> I’ve always wondered whether the “missing” antimatter in the universe is simply too far away to see, past the light horizon.

I wondered the same thing, but it doesn't work out. Rolling enough dice to get enough of the antimatter far enough away — by a combination of Heisenberg for most of it and local annihilation of what was left — is just too unlikely, given what we see. Boltzmann-brain levels of unlikely.

The issue with the "past the observable universe horizon" is that it is an entirely untestable theory. It may be true, but it may as well be irrelevant because according to our understanding of the universe, we are never going to be affected by this fact (since it's in a part of the universe from which information may never reach us).

they certainly can't hang out together for very long :)