Vera Rubin was a very brave woman. This pioneering astronomer was one of the first scientists to provide solid evidence for the existence of dark matter, one of the biggest mysteries in the entire universe.
And she did it simply by looking at the way galaxies spin.
In the 1970s, Rubin and her colleague Kent Ford were studying the rotation of spiral galaxies. They expected the outer parts of galaxies to spin slower than the inner parts, just as the planets in our solar system spin faster the closer they are to the sun.but more read here