Vera Rubin was not the first astronomer to suggest that there is invisible matter in the universe.
In the 1920s, the Dutch astronomer Jan Hendrik Oort found that the velocity of stars in our own Milky Way did not conform to Newton’s laws of gravity.
However, he thought that the problem was in his observations and did not propose any idea of dark matter.
In the 1930s, Swiss astronomer Fritz Zwicky and American astronomer Sinclair Smith reached the same conclusion.
They measured the speed of several galaxies and found them to be moving much too fast to be holding together unless there was gravity from other mass present.
They called this matter “missing mass.”
Also in the 1930s, Albert Einstein and Dutch astronomer Willem de Sitter created the Einstein—de Sitter model of the universe.
In this model, they theorized that invisible matter might exist in the universe.
They said this matter did not emit electromagnetic radiation and so had not yet been detected.