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.

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 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. During her career, Rubin examined more than 200 galaxies.