What observed property of the Milky Way suggests that it contains a large amount of matter not in the form of stars?

Question

Quizplus · Accepted Answer

The rotation curve of the Milky Way suggests that there is a large amount of matter not in the form of stars. The rotation curve shows that the velocity of stars and gas in the outer regions of the galaxy does not decrease as expected if the mass of the galaxy were mostly concentrated in the visible stars. This suggests that there must be a large amount of invisible or dark matter in the galaxy.

What Observed Property of the Milky Way Suggests That It