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

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.