The 4% Universe: Dark Matter, Dark Energy, and the Race to Discover the Rest of Reality

by Richard Panek

The problem of “missing mass” had been shadowing astronomy for decades, for almost as long as astronomers had known of the existence of galaxies. But the problem had always related to clusters of galaxies. In 1933 the Swiss-born astrophysicist Fritz Zwicky, working at Caltech, studied eight galaxies in the Coma Cluster, comparing the mass he derived from their velocities relative to one another with the mass he expected just judging from appearances. His conclusion was that the density of mass had to be four hundred times as large as what the luminosity alone suggested.* If astronomers couldn’t resolve this discrepancy, he wrote in a Swiss journal, “we would arrive at the astonishing conclusion” that the density of luminous matter in Coma must be minuscule compared with the density of some sort of dunkle—or dark —Materie—matter. Three years later, the astronomer Sinclair Smith published an article in the Astrophysical Journal about a similar pattern he’d noticed in the Virgo Cluster, suggesting the presence of “a great mass of internebular material within the cluster.” That same year, Edwin Hubble addressed the problem in his landmark book The Realm of the Nebulae: “The discrepancy seems to be real and is important.”