My understanding is that “what happened to the antimatter?” is a bit of an open question. How sure are we that all of the matter we can see is in fact matter, and not antimatter?
Antimatter has the same mass, electromagnetic spectra, falls down, etc. Do we have a way of determining which version of matter distant objects are made of?
Presumably there would be places on the outskirts where you would see the telltale gamma-ray signature of annihilation. No idea if the flux would be sufficient to observe, but I imagine a stacking analysis would see it.
Would the flux would be sufficient? Galaxies are huge, but intergalactic space is also very empty.