Experimental observation of matter/antimatter in the universe
I've heard a bit about the antimatter, matter inbalance. But I don't understand how it has been decided highly unlikely/impossible that some areas of the universe are made of matter, and others from antimatter. not necessarily specific galaxies, but maybe whole clusters/super-clusters?
Also I've also read that if this were the case then when opposite galaxies collide, it would create huge amounts of energy, which we would see, and as we don't see everything, all galaxies must be made of matter.
However when normal galaxies collide, the stars themselves do not collide with each other, so why when matter stars collide with antimatter stars would it be any different? Am I missing something like, antimatter is repelled by gravity or something?