[aufreak3]

I think the way that is usually phrased is that "photons are their own antiparticles".

Rgd "anti matter galaxies", we do detect particle cosmic rays on earth that originate from outer space (outside the solar system), though I don't know either whether they might originate from as far as other galaxies.

However, gravity is weak compared to the other forces, so galaxies can be expected to eject fast moving matter in addition to radiation (just like our sun).

So, given my meagre understanding of physics, I wouldn't expect to have seen anti-matter galaxies without also seeing gamma radiation fields spanning the space between a matter and an antimatter galaxy as particles annihilate each other.