Dwarf Galaxies could uncover the nature of Dark Matter

The circled cluster of stars is the dwarf galaxy Andromeda 29, which University of Michigan astronomers have discovered. The bright star within the circle is a foreground star within our own Milky Way galaxy. This image was obtained with the Gemini Multi-Object Spectrograph at the Gemini North Telescope in Hawaii. Credit: Gemini Observstory/AURA/Eric Bell

By University of Michigan, Ann Arbor

Published : November 7, 2011

In work that could help advance astronomers' understanding of dark matter, University of Michigan researchers have discovered two additional dwarf galaxies that appear to be satellites of Andromeda, the closest spiral galaxy to Earth.

Eric Bell and Colin Slater found Andromeda XXVIII and XXIX. They did it by using a tested star-counting technique on the newest data from the Sloan Digital Sky Survey, which has mapped more than a third of the night sky. They also used follow-up data from the Gemini North Telescope in Hawaii.

At 1.7 million light-years from Andromeda, these are two of the furthest satellite galaxies ever detected. Invisible to the naked eye, the galaxies are 100,000 times fainter than Andromeda and are barely visible even through large telescopes.

These astronomers set out looking for dwarf galaxies around Andromeda to help them understand how matter relates to dark matter, an invisible substance that doesn't emit or reflect light, but is believed to make up most of the universe's mass. Astronomers believe it exists because they can detect its gravitational effects on visible matter. With its gravity, dark matter is believed to be responsible for organizing visible matter into galaxies.

"These faint, dwarf, relatively nearby galaxies are a real battleground in trying to understand how dark matter acts at small scales," Bell said. "The stakes are high."

The prevailing hypothesis is that visible galaxies are all nestled in beds of dark matter, and each bed of dark matter has a galaxy in it.
For a given volume of universe, the predictions match observations of large galaxies.

"But it seems to break down when we get to smaller galaxies," Slater said. "The models predict far more dark matter halos than we observe galaxies. We don't know if it's because we're not seeing all of the galaxies or because our predictions are wrong."

"The exciting answer," Bell said, "would be that there just aren't that many dark matter halos. This is part of the grand effort to test that paradigm."

Detailed dark matter map yields clues to galaxy cluster growth

This NASA Hubble Space Telescope image shows the distribution of dark matter in the center of the giant galaxy cluster Abell 1689, containing about 1,000 galaxies and trillions of stars.

By STScl, Baltimore, Maryland
Published: November 12, 2010

Astronomers using NASA's Hubble Space Telescope took advantage of a giant cosmic magnifying glass to create one of the sharpest and most detailed maps of dark matter in the universe. Dark matter is an invisible and unknown substance that makes up the bulk of the universe's mass.

The new dark matter observations may yield new insights into the role of dark energy in the universe's early formative years. The result suggests that galaxy clusters may have formed earlier than expected, before the push of dark energy inhibited their growth. A mysterious property of space, dark energy fights against the gravitational pull of dark matter. Dark energy pushes galaxies apart from one another by stretching the space between them, thereby suppressing the formation of giant structures called galaxy clusters. One way astronomers can probe this primeval tug-of-war is through mapping the distribution of dark matter in clusters.

A team led by Dan Coe from NASA's Jet Propulsion Laboratory in Pasadena, California, used Hubble's Advanced Camera for Surveys to chart the invisible matter in the massive galaxy cluster Abell 1689, located 2.2 billion light-years away. The cluster's gravity, the majority of which comes from dark matter, acts like a cosmic magnifying glass, bending and amplifying the light from distant galaxies behind it. This effect, called gravitational lensing, produces multiple, warped, and greatly magnified images of those galaxies, like the view in a funhouse mirror. By studying the distorted images, astronomers estimated the amount of dark matter within the cluster. If the cluster's gravity only came from the visible galaxies, the lensing distortions would be much weaker.

Based on their higher-resolution mass map, Coe and his collaborators confirmed previous results showing that the core of Abell 1689 is much denser in dark matter than expected for a cluster of its size, based on computer simulations of structure growth. Abell 1689 joins a handful of other well-studied clusters found to have similarly dense cores. The finding is surprising because the push of dark energy early in the universe's history would have stunted the growth of all galaxy clusters.

"Galaxy clusters, therefore, would had to have started forming billions of years earlier in order to build up to the numbers we see today," Coe said. "At earlier times, the universe was smaller and more densely packed with dark matter. Abell 1689 appears to have been well fed at birth by the dense matter surrounding it in the early universe. The cluster has carried this bulk with it through its adult life to appear as we observe it today."

Mapping the Invisible

Abell 1689 is among the most powerful gravitational lensing clusters ever observed. Coe's observations, combined with previous studies, yielded 135 multiple images of 42 background galaxies.

"The lensed images are like a big puzzle," Coe says. "Here we have figured out, for the first time, a way to arrange the mass of Abell 1689 such that it lenses all of these background galaxies to their observed positions." Coe used this information to produce a higher-resolution map of the cluster's dark matter distribution than was possible before.

Coe teamed with mathematician Edward Fuselier, who, at the time, was at the United States Military Academy at West Point, to devise a new technique to calculate the new map. "Thanks, in large part, to Eddie's contributions, we have finally 'cracked the code' of gravitational lensing,” said Coe. “Other methods are based on making a series of guesses as to what the mass map is, and then astronomers find the one that best fits the data. Using our method, we can obtain directly from the data a mass map that gives a perfect fit."

Astronomers are planning to study more clusters to confirm the possible influence of dark energy. A major Hubble program that will analyze dark matter in gigantic galaxy clusters is the Cluster Lensing and Supernova survey with Hubble (CLASH). In this survey, the telescope will study 25 clusters for a total of one month over the next 3 years. The CLASH clusters were selected because of their strong X-ray emission, indicating they contain large quantities of hot gas. This abundance means the clusters are extremely massive. By observing these clusters, astronomers will map the dark matter distributions and look for more conclusive evidence of early cluster formation, and possibly early dark energy.

Hubble Provides New Evidence for Dark Matter Around Small Galaxies

These four dwarf galaxies are part of a census of small galaxies in the tumultuous heart of the nearby Perseus galaxy cluster.The galaxies appear smooth and symmetrical, suggesting that they have not been tidally disrupted by the pull of gravity in the dense cluster environment. Larger galaxies around them, however, are being ripped apart by the gravitational tug of other galaxies.
The images, taken by NASA's Hubble Space Telescope, are evidence that the undisturbed galaxies are enshrouded by a "cushion" of dark matter, which protects them from their rough-and-tumble neighborhood.Dark matter is an invisible form of matter that accounts for most of the universe's mass. Astronomers have deduced the existence of dark matter by observing its gravitational influence on normal matter, consisting of stars, gas, and dust.Observations by Hubble's Advanced Camera for Surveys spotted 29 dwarf elliptical galaxies in the Perseus Cluster, located 250 million light-years away and one of the closest galaxy clusters to Earth. Of those galaxies, 17 are new discoveries.
The images were taken in 2005.

Credit: NASA, ESA, and C. Conselice and S. Penny (University of Nottingham)

Thursday, March 12, 2009

NASA's Hubble Space Telescope has uncovered a strong new line of evidence that galaxies are embedded in halos of dark matter.

Peering into the tumultuous heart of the nearby Perseus galaxy cluster, Hubble discovered a large population of small galaxies that have remained intact while larger galaxies around them are being ripped apart by the gravitational tug of other galaxies.

Dark matter is an invisible form of matter that accounts for most of the universe's mass. Astronomers have deduced the existence of dark matter by observing its gravitational influence on normal matter, consisting of stars, gas, and dust.

The Hubble images provide further evidence that the undisturbed galaxies are enshrouded by a "cushion" of dark matter, which protects them from their rough-and-tumble neighborhood.

"We were surprised to find so many dwarf galaxies in the core of this cluster that were so smooth and round and had no evidence at all of any kind of disturbance," says astronomer Christopher Conselice of the University of Nottingham, U.K., and leader of the Hubble observations. "These dwarfs are very old galaxies that have been in the cluster a long time. So if something was going to disrupt them, it would have happened by now. They must be very, very dark-matter-dominated galaxies."

The dwarf galaxies may have an even higher amount of dark matter than spiral galaxies. "With these results, we cannot say whether the dark-matter content of the dwarfs is higher than in the Milky Way Galaxy," Conselice says. "Although, the fact that spiral galaxies are destroyed in clusters, while the dwarfs are not, suggests that is indeed the case."

First proposed about 80 years ago, dark matter is thought to be the "glue" that holds galaxies together. Astronomers suggest that dark matter provides a vital "scaffolding" for the universe, forming a framework for the formation of galaxies through gravitational attraction. Previous studies with Hubble and NASA's Chandra X-ray Observatory found evidence of dark matter in entire clusters of galaxies such as the Bullet Cluster. The new Hubble observations continue the search for dark matter in individual galaxies.

Observations by Hubble's Advanced Camera for Surveys spotted 29 dwarf elliptical galaxies in the Perseus Cluster, located 250 million light-years away and one of the closest galaxy clusters to Earth. Of those galaxies, 17 are new discoveries.

Because dark matter cannot be seen, astronomers detected its presence through indirect evidence. The most common method is by measuring the velocities of individual stars or groups of stars as they move randomly in the galaxy or as they rotate around the galaxy. The Perseus Cluster is too far away for telescopes to resolve individual stars and measure their motions. So Conselice and his team derived a new technique for uncovering dark matter in these dwarf galaxies by determining the minimum mass the dwarfs must have to protect them from being disrupted by the strong, tidal pull of gravity from larger galaxies.

Studying these small galaxies in detail was possible only because of the sharpness of Hubble's Advanced Camera for Surveys. Conselice and his team first spied the galaxies with the WIYN Telescope at Kitt Peak National Observatory outside Tucson, Ariz. Those observations, Conselice says, only hinted that many of the galaxies were smooth and therefore dark-matter dominated. "Those ground-based observations could not resolve the galaxies, so we needed Hubble imaging to nail it," he says.

The Hubble results appeared in the March 1 issue of the Monthly Notices of the Royal Astronomical Society.

Other team members are Samantha J. Penny of the University of Nottingham; Sven De Rijcke of the University of Ghent in Belgium; and Enrico Held of the University of Padua in Italy.

Caught in the cosmic-matter web

This image reveals the distribution of dark matter in the supercluster Abell 901/902, which is composed of hundreds of galaxies. The image shows the entire supercluster. NASA/ESA/C. Heymans/M. Gray/M. Barden/STAGES

January,2008

Provided by Hubble/ESA

Dark matter is an invisible form of matter that accounts for most of the universe's mass. Hubble's Advanced Camera for Surveys has mapped the invisible dark matter scaffolding of the supercluster Abell 901/902, as well as the detailed structure of individual galaxies embedded in it.

The images are part of the Space Telescope Abell 901/902 Galaxy Evolution Survey (STAGES), which covers one of the largest patches of sky ever observed by the Hubble Space Telescope. The area surveyed is so wide that it took 80 Hubble images to cover the entire STAGES field, measuring about 0.5°×0.5° on the sky. The new work is led by Meghan Gray of the University of Nottingham in the United Kingdom and Catherine Heymans of the University of British Columbia in Vancouver, along with an international team of scientists.

The Hubble study pinpointed four main areas in the supercluster where dark matter has pooled into dense clumps, totaling 1012 times the Sun's mass. These areas match the location of hundreds of old galaxies that have experienced a violent history in their passage from the outskirts of the supercluster into these dense regions. These galaxies make up four separate galaxy clusters.

"Thanks to Hubble's Advanced Camera for Surveys, we are detecting for the first time the irregular clumps of dark matter in this supercluster," Heymans says. "We can even see an extension of the dark matter toward a very hot group of galaxies that are emitting X-rays as they fall into the densest cluster core."

The dark matter map was constructed by measuring the distorted shapes of over 60,000 faraway galaxies. To reach Earth, the galaxies' light traveled through the dark matter that surrounds the supercluster galaxies and was bent by the massive gravitational field. Heymans used the observed, subtle distortion of the galaxies' shapes to reconstruct the dark matter distribution in the supercluster using a method called weak gravitational lensing. The new dark matter map is 2.5 times sharper than that from a previous ground-based survey of the supercluster."The new map of the underlying dark matter in the supercluster is one key piece of this puzzle," Gray explains. "At the same time we're looking in detail at the galaxies themselves." The survey's broader goal is to understand how galaxies are influenced by the environment in which they live.

On Earth, the pace of quiet country life is vastly different from the hustle of the big city. In the same way, galaxies living lonely isolated lives look very different from those found in the most crowded regions of the universe, like a supercluster. "We've known for a long time that galaxies in crowded environments tend to be older, redder, and rounder than those in the field," Gray says. "Galaxies are continually drawn into larger and larger groups and clusters by the inevitable force of gravity as the universe evolves."

In such busy environments galaxies are subject to a life of violence: high-speed collisions with other galaxies; the stripping away of gas, the fuel supply they use to form new stars; and distortion due to the strong gravitational pull of the underlying invisible dark matter. "Any or all of these effects may play a role in the transformation of galaxies, which is what we're trying to determine," Gray says.

The STAGES survey's simultaneous focus on both the big picture and the details can be likened to studying a big city. "It's as if we're trying to learn everything we can about New York City and New Yorkers," Gray explains. "We're examining large-scale features, like mapping the roads, counting skyscrapers, monitoring traffic. At the same time we're also studying the residents to figure out how the lifestyles of people living downtown differ from those out in the suburbs. But in our case the city is a supercluster, the roads are dark matter, and the people are galaxies."

Further results by other team members support this view. "In the STAGES supercluster we clearly see that transformations are happening in the outskirts of the supercluster, where galaxies are still moving relatively slowly and first feel the influence of the cluster environment," says Christian Wolf, an Advanced Research Fellow at the University of Oxford in the U.K.

Assistant professor Shardha Jogee and graduate student Amanda Heiderman, both of the University of Texas in Austin, concur. "We see more collisions between galaxies in the regions toward which the galaxies are flowing than in the centers of the clusters," Jogee says. "By the time they reach the center, they are moving too fast to collide and merge, but in the outskirts their pace is more leisurely, and they still have time to interact."

The STAGES team also finds that the outer parts of the clusters are where star formation in the galaxies is slowly switching off and where the supermassive black holes at the hearts of the galaxies are most active.

"The galaxies at the centres of the clusters may have been there for a long time and have probably finished their transformation," adds Heiderman. "They are now old, round, red, and dead."

The team plans more studies to understand how the supercluster environment is responsible for producing these changes.

Abell 901/902 resides 2600 million light-years from Earth and measures more than 16 million light-years across.

Dark matter found in accretion disks

January 10, 2008 (Provided by NOAO)

Observations of the interacting binary star using telescopes at Kitt Peak National Observatory and NASA's Spitzer Space Telescope suggest that the disks of hot gas that accumulate around a wide variety of astronomical objects, from degenerate stars in energetic binary systems to supermassive black holes at the hearts of active galaxies, are likely to be much larger than previously believed.

The target of this specific investigation, named WZ Sagittae (WZ Sge), is an interacting binary star located in the constellation Sagitta, the arrow of the archer Sagittarius. As part of a program called the Spitzer-NOAO Observing Program for Teachers and Students, Steve B. Howell and a team of astronomers and educators imaged WZ Sge using the National Science Foundation's 2.1-meter telescope and the WIYN 0.9-meter telescope, both located at Kitt Peak, and the Infrared Array Camera (IRAC) on Spitzer.

"We were very surprised to see the contrasting results obtained with the optical telescopes on the ground and the infrared telescope in space," says Howell, an astronomer at the National Optical Astronomy Observatory (NOAO) and leader of the team who made the discovery that was reported Wednesday in Austin, Texas, at the 211th meeting of the American Astronomical Society (AAS). "The much larger size of the infrared-emitting portion of the accretion disk around WZ Sge was immediately obvious in the data. Our observations strongly imply the presence of dark matter in these structures, which are ubiquitous throughout the universe."

Interacting binary stars such as WZ Sge contain a white dwarf star (a compact star about the size of the Earth, but with a mass near that of the Sun) and a larger, but less massive and much cooler companion star. The companion, usually a low-mass star or a brown dwarf, has material ripped off its surface by the stronger gravity of the white dwarf. This material flows toward the more massive star and, in the process, forms a disk surrounding the white dwarf, known as an accretion disk.Stars such as WZ Sge are called cataclysmic variables due to their rapid and often large changes in brightness, all caused by variations in the accretion disk. The two stars in such systems orbit about each other at a similar distance to that between Earth and the Moon, but with tremendous angular momentum that results in orbital periods ranging from a few hours down to as short as tens of minutes (the period of WZ Sge is 81 minutes).

Whether they form in cataclysmic variable systems or they surround the massive black hole hearts of active galaxies, accretion disks have been well observed and modeled using measurements obtained across much of the electromagnetic spectrum, from X-rays to the near-infrared. The derived picture of the "standard accretion disk" model is a geometrically thin disk of gaseous material surrounding the white dwarf or black hole. Accretion disk models, bolstered by observation, are generally composed of hot gas having a temperature distribution within them, being hottest near the center and falling off in temperature toward the outer edge.

In order to confirm the general accretion disk models and extend them into the mid-infrared portion of the spectrum, Howell's team obtained the first time series observations of an accretion disk system at 4.5 and 8 microns with the Spitzer Space Telescope. At nearly the same time, they obtained optical observations of WZ Sge at Kitt Peak. The optical observations confirmed the standard view of the accretion disk size and temperature, values known for over a decade.

The mid-infrared observations, however, were completely unexpected and revealed that a larger, thicker disk of cool dusty material surrounds much of the gaseous accretion disk. This outer dust disk likely contains as much mass as a medium-sized asteroid. The newly discovered outer disk extends about 20 times the radius of the gaseous disk.

"This discovery suggests that our current model for accretion disks of all kinds is wrong," says team member Donald Hoard of the Spitzer Science Center. "We will need to rethink and recast these models for accretion disks, not only in interacting binary stars but also in distant, highly luminous active galaxies."

The implications from such a discovery are far reaching, affecting not only the theoretical models (since the formation and evolution of the disks are modeled based on their size, temperature, and composition-all quantities that now need to be revised), but also nearly all previous observations of systems containing accretion disks.

In addition, the dust disk (which is thicker than the known gaseous disk) blocks infrared light emitted by the compact central object and the inner hot regions of the gaseous disk. Not knowing that some mid to far infrared light is blocked by the newly discovered outer dust ring can lead observers to significantly underestimate the total luminosity of the central object. "The amount of this underestimation is not yet accurately known from our initial discovery, but may be as large as 50 percent," Howell says.

Scientists Detect "Dark Flow:" Matter from Beyond the visible universe

photo: Galaxy clusters like 1E 0657-56 (inset) seem to be drifting toward a 20-degree-wide patch of sky (ellipse) between the constellations of Centaurus and Vela.

Credit: NASA/WMAP/A. Kashlinsky et al.

Just as unseen dark energy is increasing the rate of expansion of the universe, there's something else out there causing an unexpected motion in distant galaxy clusters. Scientists believe the cause is the gravitational attraction of matter that lies beyond the observable universe, and they are calling it "Dark Flow," in the vein of two other cosmological mysteries, dark matter and dark energy. "The clusters show a small but measurable velocity that is independent of the universe's expansion and does not change as distances increase," said lead researcher Alexander Kashlinsky at NASA's Goddard Space Flight Center in Greenbelt, Md. "The distribution of matter in the observed universe cannot account for this motion."

"We never expected to find anything like this," he said.

Using NASA's Wilkinson Microwave Anisotropy Probe's (WMAP) three-year view of the microwave background and a catalog of clusters, the astronomers detected hundreds of galaxy clusters that appear to be carried along by a mysterious cosmic flow. The bulk cluster motions are traveling at nearly 2 million miles per hour. The clusters are heading toward a 20-degree patch of sky between the constellations of Centaurus and Vela.

Several astronomers teamed up to identify some 700 X-ray clusters that exhibited a subtle spectral shift. This sample includes objects up to 6 billion light-years — or nearly half of the observable universe — away.

They found this motion is constant out to at least a billion light-years. "Because the dark flow already extends so far, it likely extends across the visible universe," Kashlinsky says.

The finding flies in the face of predictions from standard cosmological models, which describe such motions as decreasing at ever greater distances.

Cosmologists view the microwave background - a flash of light emitted 380,000 years after the big bang - as the universe's ultimate reference frame. Relative to it, all large-scale motion should show no preferred direction.

Big-bang models that include a feature called inflation offer a possible explanation for the flow. Inflation is a brief hyper-expansion early in the universe's history. If inflation did occur, then the universe we can see is only a small portion of the whole cosmos.

WMAP data released in 2006 support the idea that our universe experienced inflation. Kashlinsky and his team suggest that their clusters are responding to the gravitational attraction of matter that was pushed far beyond the observable universe by inflation. "This measurement may give us a way to explore the state of the cosmos before inflation occurred," he says.

The next step is to narrow down uncertainties in the measurements. "We need a more accurate accounting of how the million-degree gas in these galaxy clusters is distributed," says Atrio-Barandela.

"We’re assembling an even larger and deeper catalog of X-ray clusters to better measure the flow," Ebeling adds. The researchers also plan to extend their analysis by using the latest WMAP results, released in March.

The result will appear in the October 20 edition of Astrophysical Journal Letters, which is available electronically this week.

Dark Matter Ring in galaxy cluster Cl 0024+17

This Hubble Space Telescope composite image shows a ghostly "ring" of dark matter in the galaxy cluster Cl 0024+17.

The ring-like structure is evident in the blue map of the cluster's dark matter distribution. The map is superimposed on a Hubble image of the cluster. The ring is one of the strongest pieces of evidence to date for the existence of dark matter, an unknown substance that pervades the universe.

The map was derived from Hubble observations of how the gravity of the cluster Cl 0024+17 distorts the light of more distant galaxies, an optical illusion called gravitational lensing. Although astronomers cannot see dark matter, they can infer its existence by mapping the distorted shapes of the background galaxies. The mapping also shows how dark matter is distributed in the cluster.

Astronomers suggest that the dark-matter ring was produced from a collision between two gigantic clusters.

Dark matter makes up the bulk of the universe's material and is believed to make up the underlying structure of the cosmos.

The Hubble observations were taken in November 2004 by the Advanced Camera for Surveys (ACS). Thanks to the exquisite resolution of the ACS, astronomers saw the detailed cobweb tracery of gravitational lensing in the cluster.

Dark Matter

One of the biggest conundrums in modern astronomy is the fact that over 90% of the Universe is invisible. This mysterious missing stuff is known as 'dark matter'.The problem started when astronomers tried to weigh galaxies. There are two methods of doing this. Firstly, we can tell how much a galaxy weighs just by looking at how bright it is and then converting this into mass.The second way is to look at the way stars move. Everything in the Universe rotates. The Earth spins on its axis. The whole planet orbits around our parent star, the Sun. The Sun rotates around the centre of the Milky Way, along with the billions of other stars in the Galaxy, forming a huge cosmic dance. This rotation provides another way of weighing a galaxy. Studying how fast stars at the very edge move reveals the mass of the whole galaxy. The faster the Galaxy rotates, the more mass there is inside it.But when astronomers such as Jan Oort and Fritz Zwicky did the two sets of sums in the early 1930s they hit a big problem. For every galaxy they studied the two answers didn't match. They were very confident that both methods were sound as they'd been tried and tested for many years. So they came to a startling conclusion - there must be stuff out there that we just can't see - and so they called it 'dark matter'. This dark matter was really important, as if it wasn't there then galaxies would fly apart as they spun round.This might seem like a strange conclusion, but it's not really that bizarre. Imagine looking at a tower block at night. Although you can only see lights coming from some of the rooms, that doesn't mean that there aren't any more rooms in the tower. Just like these unlit rooms, dark matter can't be seen, because it doesn't shine.Astronomers are currently hunting for this missing matter. It may consist of lots of strange sounding things like MACHOs, WIMPs and neutrinos. Or there may be new solutions involving dark energy or superstring theory. But whatever it is, finding it will help to answer one of the most fundamental questions in astronomy - what is the fate of the Universe?

WIMPS :
Why are physicists looking underground to try to find the missing mass of the Universe? Well, they're searching for WIMPs. A WIMP is a Weakly Interacting Massive Particle. This is a type of 'exotic' particle that is one of the candidates for the mysterious dark matter that's missing from the Universe. Although they were formed in distant places, 'exotic' just means that they are different to the ordinary particles that make up the world around us. WIMPs have never been found, but physicists think that there could be loads of them flying straight through us all the time. In fact about a million of them could be passing through your fingernail this very second!If there are so many flying around, how come we haven't spotted one yet? Although they are called 'weak', they are amazingly strong - they can pass through solids without stopping. And they are not the only things that are zooming down from space - we are also being bombarded by cosmic rays. Both these factors make them extremely difficult to detect.But what makes WIMPs special is their ability to travel straight through solid objects. So one of the best ways to catch them is to go deep underground. The UKDMC project is housed 1100m down the deepest mine in Europe and the GENIUS team work from inside the Gran Sasso mountain in Italy. Seemingly strange locations to study the Universe, but the feature that both these places have in common is rock, tons of it. The thick rock walls act as a natural filter, so as the space particles travel downwards, the cosmic rays collide with the atoms in the rock and are halted. WIMPs should be able to pass straight through the rock and out into the caverns below where they can be detected.Teams from all around the world are currently racing to find out whether these elusive particles exist. If they do, they'll form just one of the missing pieces in the jigsaw puzzle of dark matter. It's estimated that they could contribute up to 90% of dark matter. The other main contenders for the rest are MACHOs and neutrinos.

MACHOS :
Some of the invisible dark matter that is missing from the Universe may be massive dark bodies such as planets, black holes, asteroids or failed stars (brown dwarfs). These don't produce light and so cannot be seen over great distances. [We can see planets and asteroids that lie within the Solar System because they reflect light from the Sun, but they don't make their own light like stars].All these heavyweight candidates for the missing mass are collectively known as 'MACHOs', Massive Compact Halo Objects. This is because they are large bodies that live mostly in the 'halo' of galaxies. This is like the 'outer suburbs' of the galaxy, extending from outside the 'inner city' of the galaxy's core right to the wastelands at the edge.Another possibility is that 'dark galaxies' may exist - mini-galaxies that roam undetected in intergalactic space. Around a thousand times smaller than the Milky Way, these galaxies would be too faint to see from Earth. Instead of harbouring billions of stars like our own Galaxy, they would be full of brown and black dwarfs.As with all dark matter, dark galaxies and MACHOs are both hard to detect, as we can't see them. But a surprising solution comes from the amazing effect that gravity has on the fabric of space. Everything with mass has a gravitational field, even you and me. The bigger you are, the more gravity you wield. According to Einstein's theory of General Relativity, gravity actually bends the fabric of space and the more gravity an object has, the more it warps space. This was proved by Arthur Eddington on an expedition to the Amazonian forest in 1919.This effect can be used to detect MACHOs. When a really massive object [such as a MACHO] moves in front of a distant star, its gravity bends the space around it. When light from the distant star travels through this distorted region of space it bends and becomes magnified. So while the MACHO is crossing in front of the star, it seems to move and grow in size. This is known as gravitational lensing and allows astronomers to weigh the invisible MACHO causing this phenomenon.There is no doubt that MACHOs account for some of the missing dark matter in the Universe. But there is a limit to the amount that can be attributed to them alone. This is governed by the amount of atoms formed in the Big Bang, putting a limit on the amount of ordinary matter there can be in the Universe. All in all, MACHOs could account for around 20% of the missing dark matter needed to glue the Universe together. To solve the rest of the story we have to start looking for matter that isn't ordinary, but 'exotic', such as WIMPs or neutrinos.

NEUTRINOS:
Early last century, a physicist called Wolfgang Pauli invented a particle to help his sums balance. It was a tiny elementary particle, smaller than an atom with no electric charge and no mass. All this particle did was carry energy as it zipped along at the speed of light. The problem was that even if they existed, being so small and placid these particles would be very tricky to find. Pauli himself confessed, "I have done a terrible thing. I have postulated a particle that cannot be detected."Although Pauli's particle was just made up, people took it seriously enough to integrate it into new theories in particle physics. Enrico Fermi called this enigmatic character the 'neutrino', meaning 'little neutron' in Italian. After this scientific baptism, the neutrino was accepted, even though it was still absent. The hunt was on. It took over 20 years before a particle with all the right features was spotted and the neutrino was declared alive and well.A huge amount of neutrinos was created in the Big Bang. But they are still being created in the heart of the hottest, most energetic regions in the Universe, such as supernovae, gamma ray bursts, quasars and even stars like our Sun. They even filter down to the Earth - by the time you have read this sentence an estimated 10 million neutrinos born inside the Sun will have passed through your body. When you have finished this paragraph, the same neutrinos will already be further away than the Moon.But what does this mean for dark matter, the missing mass of the Universe? Neutrinos can be considered missing because they're so difficult to detect, as they don't interact much with ordinary matter. But if they have no mass then how can they contribute to dark matter? he dark matter mystery was uncovered in the same year as Pauli 'invented' neutrinos, and being mass-less they were quickly crossed off the list of potential candidates. But a groundbreaking discovery in a Japanese zinc mine in 1998 proved everyone wrong. The Super-Kamiokande project announced that they had found that a certain type of neutrino did actually have a mass, albeit a small one, placing the particle firmly in the running as a dark matter contender.Neutrinos are known as 'hot dark matter' as they travel at the speed of light. However, they may be plentiful, but because of their extremely low mass it is estimated that only make up around 25% of the missing dark matter. So where's the rest? It's a contest between MACHOs and WIMPs.

HIGHLY STRUNG :
Currently there are two 'golden rules' of physics - General Relativity, which governs the large-scale Universe and quantum mechanics, which governs the nanoworld of atoms. The problem is that the two laws don't really agree with each other. What is needed to bridge the gap is an extra clause that links the two, a 'theory of everything', known as quantum gravity, that has yet to be discovered.Superstring theory is a contender for this prize. The idea is that the zoo of thousands of tiny 'elementary' particles that exist are not disparate entities but all originate from the same source - a vibrating string. The easiest way to imagine this is to think of a guitar string. Pressing on the fretboard alters the length of the vibrating string, producing a new note. Similarly, in superstring theory, elementary particles can be thought of as different notes played on the same string. Each string is unimaginably small, about 10²⁰ (100 billion billion) times smaller than a proton. Vibrating the string at different frequencies generates all the different types of oddly-named elementary particles, such as 'gluons', 'weakons' and 'strange quarks'.But in order to vibrate, strings need lots of room. In fact they need more room than is available in the four-dimensional world in which we live (made up of height, width, depth and time). Superstring theory requires the presence of ten dimensions! But where have the other six gone? Physicists have suggested that during the Big Bang these other dimensions were folded away, or 'compactified' leaving only four to expand and evolve.But what does this mean for dark matter, the missing mass of the Universe? If superstring theory is right then it could provide an unusual answer to this cosmic mystery. Although these hidden dimensions remain too small to be measured, gravity can travel in between them. Hence the extra mass that is missing from our Universe may just be fallout from these unseen dimensions.First we'll have to wait to see whether superstring theory is accepted as the crucial 'theory of everything'. If it is, then astronomers might have finally discovered where dark matter has been hiding out.

DARK ENERGY :
Over the past five years, the mysterious tale of dark matter has been taken yet another bizarre twist. Just as cosmologists decided that the Universe is full of a strange invisible matter, they then realised that space was even weirder than they had thought.After a full census of all the galaxies visible in the Universe, astronomers calculated that their total mass (including their hidden dark matter) only made up about one-third of the critical density needed to satisfy the best current theory about the early Universe [known as inflation].At first, cosmologists thought that inflation must be wrong. But then further data from measurements of the cosmic microwave background showed that the total density of the Universe did add up to this special critical density, so inflation could be reinstated and another solution had to be found. So even after their best attempts to renovate our cosmic home, most of the Universe still remained elusive.The problem is that this new component seems even stranger than dark matter. Not only is it invisible just like dark matter, but it must have a repulsive force, otherwise it would get sucked into galaxies and affect their motion. So this mysterious stuff has been labelled 'dark energy' and is a kind of cosmic antigravity force that counteracts the attractive force of gravity. This means that instead of the expansion of the Universe slowing down, in fact, it is speeding up. Recent measurements of distant supernovae agreed with this conclusion, finding that the Universe was indeed expanding with increasing pace.Interestingly, this had already been foreseen in a botched sum. But this wasn't the sloppy homework of a spotty student, this was a calculation by Albert Einstein no less! It was 1917 and he was trying to reduce the dynamics of the whole Universe into a set of mathematical formulae. His results insinuated that the Universe was expanding. However like the vast majority of people at the time, Einstein assumed that the Universe was static. So to stop his model Universe from growing, he invented a number he called the 'cosmological constant' to insert into his equations, restoring the safe, steady nature of his static Universe.After Edwin Hubble found that the Universe was indeed expanding, Einstein quickly retracted this number, calling it his 'biggest blunder'. But he needn't have been so hasty because now, nearly 80 years later, it has been reinstated to account for this mysterious dark energy that is overtaking gravity. These new findings are helping cosmologists look into the future and foresee the destiny of the whole Universe.

DOES DARK MATTER?

Imagine holding a black velvet bag full of ball bearings. You can feel their shape and put them on the scales to weigh them. But if you couldn't see or reach inside the bag, how would you be able to know for sure how many ball-bearings were in there?As if that wasn't difficult enough, imagine how confused you'd be if you did finally manage to examine them, only to find that there were far fewer than you originally thought. But not only that, but when you put them on the scales again they only weigh a tiny fraction of their former mass!This is similar to the conundrum facing physicists looking for dark matter - they are faced with two conflicting pieces of evidence when it comes to studying galaxies. There are two scientifically sound methods of weighing a galaxy. The problem is that they both give completely conflicting answers.Firstly, if we consider a spiral galaxy, such as our own Milky Way, or our neighbour, Andromeda, we can measure its mass by measuring how fast it spins. The faster the galaxy turns, the more mass it contains. The second way is to calculate the mass of all the observable luminous parts within the galaxy.But when physicists first made these measurements for spiral galaxies in the 1930s, they discovered that the two answers were not the same. And it wasn't a matter of a slight difference - the Milky Way was behaving as if its mass was TEN times that of the luminous component. The startling conclusion was that there must be stuff inside these galaxies that we couldn't see - 'dark matter'.The race was on to discover what this elusive dark matter actually was. If we consider the ball bearings in the bag again, the analogy of the dark matter quest is like trying to measure the ball bearings inside the bag without being able to see them directly. So how do we know what we're looking for?It's thought that the majority of dark matter is sub-atomic particles. This prediction is based on a particle physics theory called the Standard Model. This is a new theory that is attempting to unify the four fundamental forces in the Universe - the electromagnetic force (which keeps electrons bound to nuclei), the weak force (causing radioactive decay), the strong force (binding the atomic nucleus together) and gravity (which sticks everything that has mass together).The Standard Model predicts that a group of particles exists called WIMPs or Weakly Interacting Massive Particles. They tend not to interact with everyday objects, which is why we haven't spotted them so far. The rest of the dark matter could be made up of MACHOs (Massive Astronomical Compact Halo Objects). Recent data indicates that MACHOs constitute no more than 20% of the missing mass.A small percentage of the mass could also be made up from neutrinos which are now known to have mass. Or another candidate is an even more exotic particle called the axion. At the moment though, WIMPS, in particular a certain type called 'neutralinos', seem to have the best chance of filling the mass deficit.How do we look for dark matter? There are several searches taking place around the world for neutralinos using different techniques. An experiment in a laboratory near Rome, called CRESST has a small crystal of sappire supercooled to milliKelvin temperatures. When an interaction between the neutralino and the crystal takes place there should be a measurable increase in temperature. An experiment based in the United States, called CDMS, uses a combination of temperature rise and ionisation of silicon or germanium. Another experiment, DAMA in the same Gran Sasso laboratory as CRESST uses scintillation detectors. It is this type of detector that we are currently using here in the UK, as part of the UK Dark Matter Collaboration, UKDMC.All these experiments have one thing in common - they are all based underground! Our facility is based 1100m under the surface in Boulby Mine, approximately 10 miles north of Whitby on the North Yorkshire coast. The reason we conduct our experiments inside the deepest mine in Europe is to stop the detectors being bombarded by cosmic rays. These would completely mask the dark matter signal. The detectors are also constructed from the purest materials possible, as again any radiation emitted from the detector materials would swamp the signal.The scintillation detector, in this case a crystal of sodium iodide doped with thallium, emits a number of photons in direct proportion to the amount of energy deposited in the crystal by the dark matter particle. Light is also emitted when background radiation such as gamma rays and neutrons interact with the crystal. Some noise signals can be rejected online whilst others are rejected during offline analysis.Identification of the different energetic particles, including dark matter, is done by pulse shape discrimination. This is possible due to the different way the pulses interact with the target. Gamma rays will give rise to electron recoil whereas neutrons and dark matter particles will cause the nucleus to recoil. Each of the interactions gives different pulse shapes.Another way to look for neutralinos is by measuring the annual modulation of the signals from the detectors. As the earth rotates around the Sun, the Earth is either moving in the same direction as the dark matter flux (in December) or against it (in June). This is the technique utilised by the DAMA group. Preliminary results have been published which claim to show this effect, revealing the presence of neutralinos, but greater statistics are necessary to convince the scientific community that it is not just due to errors within the experiment.Here at UKDMC we have recently installed a new type of scintillation detector based on liquid xenon, called ZEPLIN I. This should improve our results considerably, as liquid xenon has a higher nuclear mass than sodium iodide (which is used inside our current detectors), making it better suited as a target for neutralinos. The problem with using xenon as a target material is that it is only liquid over a very small temperature range of four degrees, which has provided us with some interesting challenges. But controlling the target between these temperatures has been achieved and a working detector is now operational in Boulby Mine.The next stage in the ZEPLIN development is to improve the discrimination between signal (the neutralinos) and noise (other radiation). Two new detectors have been proposed and we are awaiting the birth of ZEPLIN II and III. Another detector currently under construction for deployment at Boulby uses a low-pressure gas and is called DRIFT. The beauty of this system is that it has almost 100% background noise discrimination and will be able to tell from which direction the particles are coming - the background noise should be homogeneous whilst the dark matter signal should show the annual modulation. With these advances in detector development we should soon be able to tell exactly what this elusive dark matter is and, in doing so, help solve one of the most perplexing problems in the Universe.