Nonluminous matter not directly detectable by astronomers, hypothesized to exist because the mass of the visible matter in the universe cannot account for observed gravitational effects.
Long believed to exist in large quantities, it enters into many theories of the origin of the universe and its present large-scale structure and into models of gravitation and other fundamental forces (see fundamental interaction ) between particles. Numerous candidates for dark matter have been proposed over the years, but none has yet been confirmed.