Matter that is in space but is not visible to us because it emits no radiation by which to observe it. The motion of stars around the centers of their galaxies implies that about 90% of the matter in a typical galaxy is dark. Physicists speculate that there is also dark matter between the galaxies but this is harder to verify.
It has been proposed that part of this matter might be in the form of non-baryonic matter.
Brown and black dwarfs are thought to contribute to the missing dark matter.
See also: Galaxy.