A hypothetical elementary particle that might make up most of the matter in the universe, and that is also predicted to exist in supersymmetry theory. Most matter is detected only through its gravitational effects; this “dark matter” has not been observed to emit, absorb, or reflect light of any wavelength. The total amount of dark matter appears to be approximately ten times as great as all the ordinary matter in the universe, and about one hundred times as great as all the visible matter. The nature of the dark matter is not yet known, although many experiments are under way to try to discover it directly or indirectly.