Dynamics of galactic disks

The stars in the Milky Way disk are on (almost) circular orbits, with gravity balancing centripetal acceleration. Given that most of the light of the disk comes from its central parts, we would expect the circular velocity in the outer parts of the disk to fall with distance as appropriate for Keplerian motion. We should also be able to compute how velocities of stars in the solar neighbourhood depend on direction. Observations do not follow these expectations at all, which leads to the startling conclusion that most of the mass in the Milky Way is invisible.