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 studied galaxy rotation curves. If only visible matter was considered, galaxies should spin slower at their edges. But she found they spin faster. This implies there is unseen mass (dark matter) providing extra gravitational pull.

Answer:

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