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Dark Matter - What Is It?

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Scientists using different methods to determine the mass of galaxies have found a discrepancy that suggests ninety percent of the universe is matter in a form that cannot be seen. (1). This strange material that dominates the Universe but which is invisible to current telescope technology is one of the great enigmas of modern science. That it exists is one of the few things on which researchers have been certain. What is it, really? How do we know that it's there? How do we ÐŽoseeÐŽ± it if it's ÐŽodarkÐŽ±?

Some scientists think dark matter is in the form of massive objects, such as black holes, that hang out around galaxies unseen. Other scientists believe dark matter to be subatomic particles that rarely interact with ordinary matter.

In 1933, the astronomer Fritz Zwicky while studying the motions of distant galaxies in the Coma cluster estimated the total mass of a group of galaxies by measuring their brightness. When he used a different method to compute the mass of the same cluster of galaxies, he came up with a number that was 400 times his original estimate (13). This discrepancy in the observed and computed masses is now known as "the missing mass problem. ÐŽo Nearly seventy years later his conclusion has been confirmed for many other galaxy clusters, and it is now generally believed that the most of the mass in the universe is in some dark form which has still only been detected through its gravitational effects.ÐŽ±(5)

What do scientists look for when they search for dark matter? Possibilities for dark matter range from tiny subatomic particles weighing 100,000 times less than an electron to black holes with masses millions of times that of the sun (7). The two main categories that scientists consider as possible candidates for dark matter have been dubbed MACHOs (Massive Astrophysical Compact Halo Objects), and WIMPs (Weakly Interacting Massive Particles). MACHOs are the big, strong dark matter objects ranging in size from small stars to super massive black holes (13). MACHOs are made of ÐŽoordinaryÐŽ± matter, which is called baryonic matter. WIMPs, on the other hand, are the little weak subatomic dark matter candidates, which are thought to be made of stuff other than ordinary matter, called non-baryonic matter. Astronomers search for MACHOs and particle physicists look for WIMPs. Since MACHOs are too far away and WIMPs are too small to be seen, astronomers and particle physicists have devised ways of trying to infer their existence.

MACHOs are non-luminous objects that make up the halos around galaxies. They are thought to be primarily brown dwarf stars and black holes (5). Like many astronomical objects, their existence had been predicted by theory long before there was any proof. The existence of brown dwarfs was predicted by theories that describe star formation (14). Black holes were predicted by Albert Einstein's General Theory of Relativity (8).

Astronomers are faced with quite a challenge with detecting MACHOs. They must detect, over astronomical distances, things that give off little or no light. But the task is becoming easier as astronomers create more refined telescopes and techniques for detecting MACHOs. Although the dark matter in galaxy clusters cannot be seen directly, a very direct way to measure its properties is offered by the phenomenon of gravitational lensing. The images of distant galaxies lying far behind the cluster are gravitationally distorted into giant arcs as the light passes through the cluster on its journey to telescopes. The shapes and positions of these arcs provide a detailed picture of how matter is distributed near the cluster centre (4).

In their efforts to find the missing mass of the universe, particle physicists theorize the existence of tiny non-baryonic particles that are different from what we call "ordinary" matter. Smaller than atoms, WIMPs are thought to have mass, but usually interact with baryonic matter gravitationally - they pass right through ordinary matter. Since each WIMP has only a small amount of mass, there needs to be a large number of them to make up the bulk of the missing matter. That means that millions of WIMPs are passing through ordinary matter every few seconds (11). Although some people claim that WIMPs were proposed only because they provide a "quick fix" to the missing matter problem, most physicists believe that WIMPs do exist (2).

An important ingredient in the motivation for non-baryonic dark matter comes from big bang nucleosynthesis (BBN) limits on the average baryonic content of the Universe. At the birth of the Universe, when the Big Bang occurred, the Universe was an extremely hot soup of all sorts of particles. As the Universe grew and cooled, the ÐŽoordinary matterÐŽ± particles such as Neutrons, Protons, and Electrons, started to cool enough to form atoms of the materials in the Universe that we see today - predominantly Hydrogen and Helium. BBN is an enormous success of a theory - it not only predicts that Hydrogen and Helium are by far the predominant elements in the Universe, (which is checkable, and turns out to be correct), but gets them in the right proportions. But there is a catch. The amounts of each element that forms, depends very carefully on the amount of ordinary atom-forming matter (called baryons). And BBN predicts all the right amounts for the current universe only if the original amount of baryonic matter was about §Ð© = 0.1. Note that the amount of baryonic matter is larger than the amount of visible matter; so there is some dark ordinary matter, like planets and burned-out stars. But there can't be enough to explain rotation curves and cluster velocities. (9)

High-energy physicists have proposed various candidates for non-baryonic dark matter. Neutrinos are an example of a particle that is known to exist. Early in the search for dark matter it was realized that if the neutrino had a mass in the range 10-50 eV c-2 (the electron, in comparison, has a mass of 500 000 eV c-2), then the enormous number of neutrinos created during the big bang would be able to account for all the dark matter in the universe. Although the neutrino has zero mass in the Standard Model of particle physics, various extensions of the model do allow it to have a mass. In recent years, observations of solar and atmospheric neutrinos have indicated that one flavour can change into another, which can only happen if the neutrino has mass (12).

Another candidate for non-baryonic dark matter is the family of heavier neutral particles known as WIMPs. The leading candidate in this class is the neutralino, a particle predicted by the so-called supersymmetric (SUSY) extension to the Standard Model (10). All hope of proving WIMPs exist rest on the theory that, on occasion, a WIMP will



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