[Typhon]

By the way, there's something I've been wondering about for a long time. I read somewhere that there are no antiphotons and antimatter emits light just the same as normal matter. So, I assume that there's no way to tell whether a photon was emitted by antimatter or by matter.

Therefore, how do we know distant galaxies aren't made of antimatter ?

Maybe there's an obvious answer, but as far as I know, the only thing we can know about these galaxies, we know because of the radiation they emit.

[InclinedPlane]

Empty space isn't completely empty, even between galaxies. There are traces of gas, almost entirely Hydrogen, between galaxies. If there were entire galaxies made of anti-matter then the areas where their clouds of inter-galactic anti-gas contacted normal inter-galactic gas would be rife with emissions due to annihilations. The amount of matter should be enough to be detectable even at tremendous distances.

[othermaciej]

I've wondered this myself. An explanation I have heard from a physisicist is: if there were regions of the universe where antimatter is dominant, the boundaries where they border normal matter space would be extremely bright from the annihilation reactions. Apparently even very thin interstellar gas would generate a lot of energy in matter/antimatter annihilations.

[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.

[ww520]

If antimatter are repulsed by gravity, they might be in the outer shell of the universe being pushed further away. If antimatter also repulsed among themselves (??), then they won't have a chance to come together to create antimatter star.

Most people believe antimatter attract just like matter, so it's a moot point.