Annu. Rev. Astron. Astrophys. 1979. 17: 135-87
Copyright © 1979 by . All rights reserved

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With the recognition of galaxy clusters as dynamical entities, it was realized that the internal kinematics could be used to measure average masses of galaxies. Zwicky (1933) and Smith (1936) applied the virial theorem to the Virgo cluster using the small sample of radial velocities then available and found that the virial mass exceeded by a factor of several hundred that expected from the sum of the masses of individual galaxies. In a classic paper Zwicky (1937) derived an M / LB of 500 for the Coma cluster which he compared with the M / LB appeq 3 for the solar neighborhood. Thus the ``missing mass,'' or more properly ``invisible mass,'' problem was born. Since then, our understanding of the structure and properties of clusters of galaxies has grown enormously (see reviews by Abell 1975, van den Bergh 1977, Bahcall 1977). However, the problem of invisible mass in the great regular clusters has not abated.