A way of estimating the total mass of an object such as a galaxy or a cluster of galaxies from the movement of its individual members. The theorem states that the average gravitational potential energy of the constituent objects is twice their average kinetic energy. Calculations with the virial theorem show that galaxies and clusters contain up to ten times as much mass as can be seen telescopically, providing strong evidence for the existence of large quantities of dark matter. A modified version of this theorem, called the cosmic virial theorem, applies on cosmological scales. It relates the statistics of galaxy motions and the correlation function (which describes the way galaxies cluster in space) to the average density of the Universe. Since the first two quantities are measurable, the density parameter can thus be estimated. The usual result obtained is around 0.2, indicating that there is dark matter on cosmological scales, but not enough to reach the critical density.