Question

which discovery by astronomer vera rubin helped prove the existence of dark matter? galaxies exist outside of the milky way. the expansion of the universe is accelerating. galaxies spin faster than they should, based on their visible matter. gas giants form farther from the sun, where temperatures are cooler.

Explanation:

Vera Rubin's observations of galactic rotation curves showed that stars in the outer regions of galaxies moved much faster than could be accounted for by the gravitational pull of visible matter alone. This discrepancy provided key evidence for the existence of unseen, massive dark matter that exerts additional gravitational force. The other options are unrelated to her work: galaxies outside the Milky Way were discovered by Edwin Hubble, accelerating cosmic expansion was found by other teams, and gas giant formation is part of solar system formation theory.

Answer:

Galaxies spin faster than they should, based on their visible matter.