|MadSci Network: Astronomy|
One of the main issues in cosmology is whether the universe has sufficient mass to cause its expansion to gravitationally slow down, stop and eventually recollapse to what is known as the Big Crunch. If there is not sufficient mass, the universe could simply continue to expand indefinitely. If we look at all the luminous matter in the universe (stars and galaxies) there does not appear to be enough to cause the universe to recollapse. But we also know that non-luminous or dark matter exists in the universe. The reason we know this is that we live on some of it. The Earth is an example of dark matter. There is nothing special or exotic about dark matter--the term just refers to matter in the universe that isn't luminous. The question is, since we can only see nearby dark matter, how do we find out how much there is in the universe. One way is to look for gravitational microlensing--the focusing of distant starlight by nearer dark objects. This is a good way to look for Massive Compact Halo Objects (MACHO). Jupiter and the other planets are examples of MACHOs, so we know at least some exist--and there is growing evidence for others (see reference below). Other possibilities for dark matter are, as you say, WIMPs (Weakly Interacting Massive Particles) and Neutralinos, predicted by supersymmetry theory. There's a lot of information on these topics at the MIT Net Advance of Physics pages at: http://web.mit.edu/afs/athena.mit.edu/user/r/e/redingtn/www/netadv/dkmat ter.html [You may also find it useful to look at the Centre for Particle Astrophysics page on dark matter.]
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