What is dark matter, and what are the problems with it?
Dark matter may sound very mysterious, but it is simply a name which astronomers give to any stuff in the universe which we can detect gravitationally but not "see". In other words, for many possible reasons, the material is not emitting light of any wavelength which we can detect, but we can measure its gravitational effect on other objects in the universe.
The first evidence of dark matter was found in clusters of galaxies back in the 1930's. Astronomer Fritz Zwicky discovered that the mass of luminous material in a cluster of galaxies (i.e. the galaxies themselves and any gas which was detected) was much less than the total mass of the cluster implied by the velocities of the galaxies. Evidence of dark matter has since been found within galaxies, and it appears that more than 90% of the total mass of the universe may be dark matter.
The search for dark matter continues. Some dark matter is in the form of 'brown dwarfs', 'black dwarfs', and planets, which we know exist but which are generally too faint to be detected other than by their gravitational effect. It seems unlikely, however, that these dim objects can account for all of the missing mass. There are many other candidates, ranging from as yet undetected exotic particles to black holes. Stay tuned to 'The Universe' for more on dark matter.
Submitted by Ron (Israel)
(March 13, 1998)