Comment Re:Matter, anti-matter... (Score 1) 393
One possibility I have been wondering about is that of antimatter galaxies. Seen from a distance, wouldn't an antimatter galaxy look exactly like one made of matter? I have been told this is not a possibility either, since that would imply that somewhere there would have to be a boundary between matter and antimatter, where a lot of annihilation would be going on and producing gamma-radiation, which we have not observed.
I believe the explanation comes before that. During the primordial nucleosynthesis when matter and anti-matter were being created, they would be so well mixed that it would be impossible for galaxies to form. For any clumps of matter to form, the universe would have to be much less dense than it is now, and those clumps would not nearly be in the size of galaxies.