Newtonian and Einsteinian formulations for the gravitational forces exerted by galactic cores on the stars in the spiral arms is proportional to the square of the distance between them. Accordingly, the speed at which these stars revolve around the core should be proportional to the square root of the radius, and should drop off as this distance increases. However, it is observed that the speed remains almost constant along the whole spiral arm.
Mainstream science accounts for this motion by postulating that galaxies are surrounded by enormous haloes of dark matter. It has no credible explanation as to what this dark matter should consist of, nor can it say why the haloes have the form and position which is required for them to have the required effects.
QO posits that this motion is caused by local variations in the scale factor. See lemma /explanations/galactic-rotation.