Which discovery by astronomer Vera Rubin helped prove the existence of dark matter?

Vera Rubin measured how quickly stars and gas move around the centers of galaxies. The outer parts of galaxies rotated much faster than the gravity from their visible matter could explain, suggesting that unseen mass—dark matter—was present. This matches choice C, galaxies spin faster than they should based on their visible matter.

AI-written and checked against the verified answer.