My personal idea/hope is that there is some other dimension of spacetime over which the big bang had directionality, emitting matter and antimatter across different poles, and that's why. That'd also mean there's an anti-universe, which is why I like the idea.

In terms of the galaxies, I believe there's enough of an observable difference that I think we would be able to detect antimatter clusters, or similar, based on emission lines but I'm not 100% on that. Huge annihilation events from colliding galaxies and clusters would have massive energy signatures unlike anything else but the frequency of this would determine how likely it would be to see the evidence.