© M33 Image: NOAO, AURA, NSF, T.A.Rector.
The green points on the plot correspond to the observed velocities of objects orbiting the M33 galaxy as a function of their distance from the galactic center. The lower curve on the plot (dashed line) shows what the rotational velocity of objects in the M33 galaxy is expected to be based on the luminous matter in the galaxy. Clearly, the green points do not match the dashed line: the rotational velocity of objects outside the galaxy is far faster than the prediction. If, however, there were a large amount of non-luminous matter in the galaxy, objects far from the galactic center would move much faster. The solid green line is the velocity predicted for the orbiting objects if there is dark matter in M33. These rotation curves provide strong indirect evidence for dark matter. (Unit: 10)