Summary

German physicist Fritz Zwicky observed in 1933 that the visible mass of a galactic cluster of stars could not be held in balance only by the observable matter which emits light. There would need to be 400 times the amount of visible matter to produce sufficient gravitational pull to hold these clusters. Zwiucky named this unknown force dunkle Materie ("dark matter"). These observations remained scientific oddities until the 1970s when scientists began to probe further into the question of dark matter. Astronomers Vera Rubin and Kent Ford found that the rotational speeds of stars were identical regardless of their distance from the galactic center. They concluded that ordinary, visible matter accounted for only one-sixth of the mass required to hold them in stable orbits. Where did the additional mass come from? According to Ruben and Ford, additional mass came from...