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 observed property of the Milky Way that suggests the presence of a large amount of matter not in the form of stars is the rotation curve. The rotation curve of the Milky Way shows that the orbital velocities of stars and gas remain relatively constant with increasing distance from the galactic center, instead of decreasing as expected based on the mass of visible matter. This suggests the presence of a large amount of invisible, or dark, matter that is exerting gravitational forces on the stars and gas.

What Observed Property of the Milky Way Suggests That It