Vera Rubin was an American astronomer who pioneered work on galaxy rotation rates. She found a discrepancy between the predicted angular motion of galaxies and the observed motion, by studying galactic rotation curves. Basically she found that things in far reaches of a galaxy rotated at the same speed as things near the center, an unexpected phenomenon called the galaxy rotation problem. A possible explanation was dark matter, that there was more matter in a galaxy than anyone could see. So she is best known for discovering evidence of dark matter. She transformed modern physics and astronomy and was awarded the National Medal of Science in 1993 by President Clinton for her work.