Vera Rubin, Astronomer

It seems a pity to only find out about people like this when they die.
Vera Rubin was an astronomer, who found evidence for the existance of dark matter thanks to her work on mapping the motion of stars within spiral galaxies. She found that the stars in the outer regions of the spiral move at about the same speed as the ones in the middle, leading to the hypothesis that most of the universe is filled with "dark matter", which we cannot detect directly. In the solar system, the outer planets move more slowly than the inner planets. According to general relativity, the stars should shoot off into space, but some unseen matter must be providing extra gravitational pull to cause them to stay within the galaxy.

During her career, Princeton declined to send her a course catalogue because they didn't accept women (she went to Cornell, where she met her husband), she had to talk to the astrophysicist George Gamow in the lobby of the building because they would not allow women in the offices, and when she became the first woman to be given access to the Palomar Mountain 200 inch telescope in California, she found there was no women's toilet.
She never won a Nobel prize, but she did win a gold medal from the Royal Astronomical Society - the first woman to do so since Catherine Herschel in 1828.