Discovery of Dark Matter
Dark matter came under discussion because parts of the universe seemed to be misbehaving. During the 1930s, Fritz Zwicky, a Swiss astronomer, was measuring orbiting speeds of a cluster of galaxies over 320 million light-years from earth.
The fact that the galaxies were orbiting at all was strange. Based on the amount of mass that could be measured, they shouldn’t have been clustered. The star systems should have moved away from each other long, long ago.
Zwicky theorized that an unknown, unseen type of matter, which he referred to as “dark," allowed the galaxies to continue orbiting. Two decades later, astronomers Vera Rubin and Kent Ford, working at Washington D.C.’s Carnegie Institution, contributed to this theory.
Rubin and Ford began with the assumption that most of a spiral galaxy’s mass is in its core, since that’s the location of most of the visible matter. That means that a galaxy’s gravitational pull would be strongest at the center, as well.
But their observations didn’t support this. Stars at the outer edges of a galaxy should have been moving more slowly, because the gravitational pull was weaker.
However, stars throughout a spiral galaxy moved at the same speed. To explain this, Rubin concluded that galaxies have about 10 times as much hidden mass as visible mass.