 Until the early part of the 20th century, it went without saying that the matter we see is most of the matter there is. That would be protons and neutrons with accelerating electrons creating the light we see. But that came into question in the early 1930s when Fritz Swicke, a Swiss astronomer out of Caltech, studied the coma cluster 321 million light-years away with a thousand galaxies spanning 25 million light-years in diameter. He looked at it in a number of ways, two of which are very revealing. In one, he used galaxy motion to calculate mass and in the other, he used galaxy luminosity to calculate mass. His processes are not precise but they do provide ballpark figures for the mass of the cluster. For motion, he had the cluster galaxy's radial velocities from the Doppler shift in the light we see. He then generalized them into their three-dimensional velocity dispersion statistical equivalent. This galaxy motion gives us the kinetic energy for the cluster. Swicke used the well-understood virial theorem that has the kinetic energy of a system equal to one-half its gravitational potential energy. This allows us to solve for the mass of the cluster. This is the mass as measured by its gravitational effects. The second way he calculated the cluster's mass was to use the cluster's luminosity. You may recall from our discussion on the Hertzsprung-Russell diagram in our How Far Away Is It Segment on Distant Stars that there is a relationship between a star's mass and its luminosity. We can use that relationship to estimate the mass of groups of stars by measuring their luminosity. We use the mass to light ratio of the sun as the base for comparisons. Swicke measured the luminosity of the average galaxy in the Coma cluster. Using a mass to light ratio of three, he calculated its mass. When he multiplied the average times a thousand galaxies in the cluster, he came out with a number that was over a hundred times less than the mass calculated via the virial theorem based on gravity. In other words, the motion of the galaxies in the cluster indicated a mass that was over a hundred times the mass from luminous matter. Swicke concluded that either the laws of gravity as we know them, Newtons and Einstein's, did not work for volumes as large as the Coma cluster. Or the luminous matter is only a very small part of the total matter of the cluster. He called the rest of the matter dark matter and suggested that gravitational lensing could help quantify this dark matter. But back in the 1930s, nobody believed him.