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The statement that, in a self-gravitating system in equilibrium (e.g. a cluster of stars or galaxies), the gravitational potential energy is twice the kinetic energy of the member objects. From this theorem, the masses of galaxy clusters can be estimated from their size and the average velocities of their luminous members. The values obtained are nearly ten times greater than the total mass of visible galaxies, and this has given rise to the problem of missing mass in galaxy clusters. It indicates that there are large quantities of dark matter in clusters of galaxies.
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