PRINCETON, N.J. – Vera Rubin, a pioneering astronomer who helped find powerful evidence of dark matter, has died, her son said Monday.

She was 88.

Allan Rubin, a professor of geosciences at Princeton University, told The Associated Press his mother died Sunday night of natural causes. He said the Philadelphia native had been living in the Princeton area.

Vera Rubin found that galaxies don’t quite rotate the way they were predicted, and that lent support to the theory that some other force was at work, namely dark matter.

Dark matter, which hasn’t been directly observed, makes up 27 percent of universe – as opposed to 5 percent of the universe being normal matter. Scientists better understand what dark matter isn’t rather than what it is.

Rubin’s scientific achievements earned her numerous honors, including becoming the second female astronomer to be elected to the National Academy of Sciences. She also received the National Medal of Science from President Bill Clinton in 1993 “for her pioneering research programs in observational cosmology.”

Rubin’s interest in astronomy began as a young girl and grew with the involvement of her father, Philip Cooper, an electrical engineer who helped her build a telescope and took her to meetings of amateur astronomers.

She was the only astronomy major to graduate from Vassar College in 1948. When she sought to enroll as a graduate student at Princeton, she learned women were not allowed in the university’s graduate astronomy program, so she instead earned her master’s degree from Cornell University.

Rubin earned her doctorate from Georgetown University, where she later worked as a faculty member for several years before working at the Carnegie Institution in Washington., a nonprofit scientific research center.

During her career, Rubin examined more than 200 galaxies.