(from a posting I made to sci.space.science, Sept 2, 1998)

  Alex asked: 

> I was wondering if anybody was curious about Dark Matter theories
> because I need help in defining what it is.
> Can anybody give me some clues please?
> --PS what are MACHOS and WHIMPS?

  "Dark matter" is the name given to the material in our universe
whose existence we infer from its gravitational pull, but not from
any directly observed photons.  So, for example, we see a galaxy, and
we measure the light coming from it: we might guess it contains
100 billion stars; we might further guess that the total mass of
the stars might be 10 billion solar masses.  But when we analyze
the motions of the stars in the galaxy, we find that there must
be perhaps 100 billion solar masses of material in it.

  The difference between the visible and the inferred mass,
 
          100 billion - 10 billion = 90 billion solar masses

we explain by saying, "Gosh.  There must be 90 billion solar masses
of some sort of Dark Matter in that galaxy."

  We don't know what the Dark Matter is.  It is probably a mix
of several different components.  Some candidates are:

           - cool, dim stars less massive than our sun
           - planets, big or small
           - bricks, snowballs
           - gas in a narrow range of temperatures around a million
                  degrees (where its radiation in the far UV would be
                  hard for us to detect)
           - neutrinos (if they have mass)
           - wierd new elementary particles
             
  "MACHO" = Massive Compact Halo Object is just another name for one
type of candidate: small, dim stars or big, Jupiter-like planets.
"WIMP" = Weakly Interacting Massive Particle is just another name
for an entry from the last category.

  Lots of people are looking for stuff in all the above categories.
So far, no one has found enough of any component to account for all
the gravitational effects we can observe.

  You can find more information about Dark Matter in just about
any recent popular astronomy book.   My poor memory recalls authors
such as Timothy Ferris, Don Goldsmith, Rocky Kolb, among many others.
Just visit your local bookstore and browse through the šPhysics and
Astronomy" section.