What observed property of the Milky Way suggests that it must contain a large amount of matter not in the form of stars, gas, or dust?

Question

Quizplus · Accepted Answer

The rotation curve of the Milky Way, which plots the orbital velocity of stars and gas against their distance from the galactic center, remains flat at large radii. This suggests the presence of a large amount of unseen mass, or dark matter, beyond the visible components like stars, gas, and dust.

What Observed Property of the Milky Way Suggests That It