Re: Dark Matter

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.]