When the Universe was young, it was nearly smooth and featureless. As it grew older and developed, it became organized. We know that our solar system is organized into planets (including the Earth!) orbiting around the Sun. On a scale much larger than the solar system (about 100 million times larger!), stars collect themselves into galaxies. Our Sun is an average star in an average galaxy called the Milky Way. The Milky Way contains about 100 billion stars. Yes, that's 100,000,000,000 stars! On still larger scales, individual galaxies are concentrated into groups, or what astronomers call clusters of galaxies.
The cluster includes the galaxies and any material which is in the space between the galaxies. The force, or glue, that holds the cluster together is gravity -- the mutual attraction of everything in the Universe for everything else. The space between galaxies in clusters is filled with a hot gas. In fact, the gas is so hot (tens of millions of degrees!) that it shines in X-rays instead of visible light. In the image above, the hot X-ray gas (shown in pink) lying between the galaxies is superimposed on an an optical picture of the cluster of galaxies. By studying the distribution and temperature of the hot gas we can measure how much it is being squeezed by the force of gravity from all the material in the cluster. This allows scientists to determine how much total material (matter) there is in that part of space.
Remarkably, it turns out there is five times more material in clusters of galaxies than we would expect from the galaxies and hot gas we can see. Most of the stuff in clusters of galaxies is invisible and, since these are the largest structures in the Universe held together by gravity, scientists then conclude that most of the matter in the entire Universe is invisible. This invisible stuff is called 'dark matter'. There is currently much ongoing research by scientists attempting to discover exactly what this dark matter is, how much there is, and what effect it may have on the future of the Universe as a whole.
The dark matter component has vastly more mass than the "visible" component of the universe. At present, the density of ordinary baryons and radiation in the universe is estimated to be equivalent to about one hydrogen atom per cubic metre of space. Only about 4% of the total energy density in the universe (as inferred from gravitational effects) can be seen directly. About 22% is thought to be composed of dark matter. The remaining 74% is thought to consist of dark energy, an even stranger component, distributed diffusely in space. Some hard-to-detect baryonic matter makes a contribution to dark matter, but constitutes only a small portion. Determining the nature of this missing mass is one of the most important problems in modern cosmology and particle physics. It has been noted that the names "dark matter" and "dark energy" serve mainly as expressions of human ignorance.
Detection of Dark Matter
These cosmological models predict that if WIMPs are what make up Dark Matter, trillions must pass through the Earth each second. Despite a number of attempts to find these WIMPs, none have yet been found.
Experimental searches for these dark matter candidates have been conducted and are ongoing. These efforts can be divided into two broad classes: direct detection, in which the dark matter particles are observed in a detector; and indirect detection, which looks for the products of dark matter annihilations. Dark matter detection experiments have ruled out some WIMP and axion models. There are also several experiments claiming positive evidence for dark matter detection, such as DAMA/NaI and EGRET, but these are so far unconfirmed and difficult to reconcile with the negative results of other experiments. Several searches for dark matter are currently underway, including the Cryogenic Dark Matter Search in the Soudan mine and the XENON experiment at Gran Sasso, and many new technologies are under development, such as the ArDM experiment.
One possible alternative approach to the detection of WIMPs in nature is to produce them in the laboratory. Experiments so far with the Large Hadron Collider near Geneva, suggest that WIMPs are at least 100 times more massive than the proton. Some have suggested as a result, that WIMPs may be the heavier counterparts of known particles predicted by supersymmetry theories. Such massive particles interact only weakly with ordinary matter.
The Cryogenic Dark Matter Search, in the Soudan Mine in Minnesota aims to detect the heat generated when ultracold germanium and silicon crystals are struck by a WIMP. The Gran Sasso National Laboratory at L'Aquila, in Italy, use xenon to measure the flash of light that occurs on those rare occasions when a WIMP strikes a xenon nucleus. The results from April 2007, using 15 kg of liquid and gaseous xenon,failed to detect any, and in March 2008 the team is starting again using 150 kg of the material. For every hundred trillion WIMPs they expect to detect 1-10 flashes per year. The GLAST space telescope, planned for launch in October 2008, searching gammawave events, may also dectect WIMPs. WIMP supersymmetric particle and antiparticle collisions should release a pair of detectable gamma waves. The number of events detected will show to what extent WIMPs comprise Dark Matter.
With all these experiments together, scientists are becoming confident that WIMPs will be discovered in 2008. But scientists are beginning to think that Dark Matter is composed of many different candidates. WIMPs may only be a part of the solution.