The rotation curve of a galaxy is a graph showing the galaxy's speed of rotation at different distances from the center. The observed rotation curve in the outer parts of a typical large spiral galaxy
A) is quite flat (roughly the same speed at all distances).
B) decreases suddenly to zero at the outer edge of the visible galaxy.
C) decreases smoothly with increasing distance from the center.
D) increases drastically with increasing distance from the center.

Question

Quizplus · Accepted Answer

The observed rotation curves of large spiral galaxies remain roughly flat with increasing distance from the center, indicating that the speed of stars and gas in the outer parts of the galaxy does not decrease as expected with distance. This observation is one of the main pieces of evidence for the existence of dark matter.

The Rotation Curve of a Galaxy Is a Graph Showing