During the early part of this century, astronomers believed that the distribution of mass in the universe followed the distribution of light. In other words, more light meant more stars, which in turn meant more mass. Unfortunately, things aren't that simple. In 1938, Fritz Zwicky and Sinclair Smith discovered a cluster of galaxies that, according to theory, shouldn't exist.
According to Kepler's third law of planetary motion, the speed (linear, not angular) of a body revolving around another decreases as the distance between them increases. This is evidenced by the speed of the planets revolving around the Sun; Mercury moves at 48 km/sec, while Pluto's speed is 4.7 km/sec. This law also applies to stars revolving around the galactic core and galaxies revolving around each other.
What Zwicky discovered was a group of galaxies that was spinning too fast. In theory, the cluster should have disassociated. In the late 1970s, Vera Rubin conducted a study on the rotation of galaxies. The velocity of the stars in a galaxy was measured using a radio telescope. If you look at a spiral galaxy edge-on, some stars will be moving toward you and some will be moving away because of the rotation of the disk. Heated hydrogen gas emits radio noise at a certain well-defined wavelength. The hydrogen signatures of these stars will be Doppler shifted to an extent determined by their speed. By plotting the velocity (calculated from the Doppler shift) against the distance from the galactic core, a rotation curve can be constructed. What Rubin found was that a large portion of the graph was flat; the stars near the edge of the galaxy were moving at the same speed as stars in the middle.
As it turns out, computer models indicate that this behavior can be explained if there is a large amount of invisible mass surrounding the galaxy. This halo of "dark matter" would keep the orbital velocities more constant as well as make the galactic disk more stable. So what could this dark matter be? Many scientists believe that it is made up of neutrinos. Neutrinos are extremely abundant. In fact, several hundred trillion of them will have passed through your body by the time you finish this sentence. They are generated by such things as the Big Bang and the nuclear processes that occur within stars. Neutrinos are extremely difficult to find because they rarely interact with matter. Although they are so abundant, only a handful are detected in traps every year. Unfortunately, nobody knows with certainty if neutrinos are massive. However, if they have just a tiny mass (one thirty-thousandth the mass of an electron), they could account for the dark matter.
Cosmologists are also interested in dark matter for another reason. Alan Guth's inflation model (which explains why the universe exploded during the Big Bang) demands that the universe is flat. That is, the rate of expansion must be the smallest possible value that will avoiding the eventual collapse of the universe. The rate of the expansion of the universe depends on its mass. So far, only approximately fifteen percent of the necessary mass has been accounted for. It is thought that the remaining mass may be found in dark matter.
Dark Matter - thorough description of dark matter and possible types
Dark Matter - overview, ramifications of dark matter
Dark Matter, Cosmology... - neat galaxy cluster simulation (Java)
Dark Matter Links - the different possible types of dark matter
* Photo credit - ESA, NASA, R. Mushotzky