Why don't I just ramble on with another post.... Ahem.

The Milky Way is very flat and thin. It has a mass of approximately 6 * 10^11 solar masses (including dark matter), or about 11.934e+41 kg. It has a radius of approximately 60,000 lightyears, or 5.676E+20 meters. Therefore it could be argued that the area represented by gravitationally bound matter is about 7.028E+31 square meters. The Milky Way disk has a thickness of about 2000 lightyears, or 1.892E+19 meters, so the volume is about 1.329E+51 cubic meters. So the density of the Milky Way galaxy is about 8.974E-10 kg/cu.meter. In terms of materials this makes for a very lightweight solar sail, far lighter than any electromagnetically bound material.

Radiation impinging on particles that are gravitationally bound to the galactic core would transfer momentum. If we assume that there is some net directional radiation in the universe, then the Milky Way would get pushed away from other galaxies. If the Milky Way also increased its diameter with time, and the intensity of radiation was constant, then it's acceleration would be constant. Alot of assumptions I know.