A major reason for believing in the existence of dark matter is that the stars in the outer regions of galaxies are moving too fast to remain bound to their galaxies, given only the estimated mass of the luminous matter.
But what if many of these stars are not actually bound, and really are in the process of escaping the galaxies?
The loss of stars in the outer portions of galaxies at some stage in their evolution could also be a possible explanation for the "Cosmic Downsizing" mentioned in another recent question.
Given the extremely long periods of rotation, and the comparatively short length of time that we have been observing them, it must be very difficult to determine any rates of change of galactic rotations, so how sure can we be that they have reached any sort of equilibrium?