The additional, unseen matter whose gravitational effect is needed to explain the rotation speeds of galaxies, and also to keep clusters of galaxies together. Such mass is thought to reside in massive dark haloes around galaxies. In addition, if gravity is to prevent the Universe from expanding indefinitely, there must be considerably more matter in intergalactic space than is contained in visible stars and galaxies. The term dark matter is now generally used instead of missing mass.
Subjects: Physics — Astronomy and Astrophysics.