The matter in the universe that has so far escaped our detection. The presence of this unseen matter has been inferred from its gravitational influence on the motions of stars and gas in galaxies and the behavior of galaxy clusters.
A type of matter which interacts with other matter only by gravitation. We infer the existence of dark matter from motions of stars and gas in spiral galaxies and from the velocities of galaxies in great clusters. Candidates for dark matter include the mundane (burnt out stellar remnants) to the bizarre ("primordial" black holes or new elementary particles).
Dark matter is any matter in the Universe which does not give off any light of its own, or does not interact with light the way typical matter does. Check out these CfPA dark matter links: The question of dark matter The WIMP experiment The MACHO experiment The CMB Theory Group Big bang cosmology primer