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.
The universe expands because quantum fluctuations continually produce new space quantum systems. It expands into itself locally, in such a manner that surrounding space is not directly affected.
Phenomena explained :
Galactic rotation curves: The velocity at which stars in the spiral arms of galaxies revolve around galactic cores is more or less constant, regardless of the radius.