The history of dark matter dates back to the 1930's when Fritz Zwicky
at Mount Wilson Observatory, who found that cluster of galaxies did not
contain enough matter to be gravationally bound. If they are bound then some
of the mass is missing. In the 1970's Vera Rubin extended this missing mass
to galaxies themselves by showing that the dynamically galaxies had to have
more mass in their outer halo then was observed by any known technique.
Approaching this problem from the other direction, we don't see
"detect" enough matter in the universe to make it closed (the expansion will
eventully stop and collapse) by about two orders of magnitude with an error
of atleast plus or minus one order of magnitude. Also the counting method
is biased in that it only counts what we can detect, and makes one wonder
why it comes out so close to being closed or right at the dividing line.
Particle physists realized a little over a decade ago that if
neutrinos have rest mass, then a great many of them must be floating around
in clouds leftover from the creation, or if not neutrinos then what about
photonios, axions or other particles that may have been made in the early
universe. These could well makeup an undetectable majority of the mass of
the universe.
Finally the inflationary theory of the universe requires that the energy of expansion be equal to the gravitational potential energy, that is there is
more matter in the universe then we have detected.
There is missing mass or dark matter in the universe but it may well
not be enough to accound for all the "problems" sighted above.
Gary