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 the presence of a large amount of matter that is not in the form of stars. This is because the observed velocities of stars at different distances from the center of the galaxy do not decrease as expected based on the visible mass (stars and gas) in the Milky Way. This suggests the presence of invisible matter, also known as dark matter, which is thought to make up about 85% of the total matter in the galaxy.

What Observed Property of the Milky Way Suggests That It