rotation curve

For a galaxy, the rotation curve is a graph showing how the measured orbital velocity of stars and interstellar matter varies with distance from the center of the galaxy. A Keplerian rotation curve would indicate that most of the mass was concentrated toward the center. However, in many cases, the rotation curve is "flat"; in other words, the rotational velocity of the galaxy remains constant with increasing radius. This suggests that large amounts of dark matter exist beyond the visible region.