In 1933 Fritz Zwicky was the first to find a need to invoke the idea of missing mass or dark matter. He looked at eight Coma galaxies. By assuming visual equilibrium,* he calculated the mass-to-light ratio and determined that about 90% of the mass necessary to account for the observed ratio was missing and therefore invisible. or “dark.” Here, the apparent rapid velocities of the galaxies, with respect to their common center of mass, suggested that much more mass (than could be seen) was required to keep the galaxies from flying out of the cluster. (BSVD) *Visual equilibrium means that the amount of light (of stars) is proportional to amount of mass (number of stars).

In 1936 Sinclair Smith found similar evidence for the existence of invisible mass in the Virgo cluster.

In 1940 Oort estimated (based on the Mass-to-light ratio of spiral nebulae) that 90% of the mass in the local group of spiral nebulae is “missing.” Oort didn’t cite Zwicky’s 1933 paper (BSVD)…

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