Coma Cluster

Abell 1656, a very rich cluster ~320 million light-years away in Coma Berenices, containing more than 1000 known galaxies dominated by two giant ellipticals NGC 4874 and NGC 4889. Fritz Zwicky's 1933 measurements of its galaxy motions provided the first observational evidence for dark matter — the galaxies move too fast to be held together by their visible mass alone.