Why does the lack of antimatter in the observable universe suggest a baryon asymmetry? My title question relies on the answer to this question: Under the standard model and assuming baryon symmetry, is it possible (however unlikely) that some small local pocket of the universe might have had a little more surviving matter than antimatter?
If that's possible, than by the weak anthropic principle, it seems to me that we can't take a local imbalance as evidence of a fundamental asymmetry. Such an unlikely event is bound to happen countless times in a sufficiently large universe - and we have no reason to believe that we don't live in a sufficiently large universe. 
Because we could only have come to exist in one of those rare, unlikely pockets with a local imbalance, we can't use that local imbalance as evidence of anything global. It may just as well be seen instead as evidence that the universe is large enough to have such pockets.
The observable universe might be profoundly unusual by the standards of the universe as a whole... but in the parts of the universe that had a more equal distribution of matter and antimatter, there are no planets or stars, and certainly no intelligent life present to wonder about symmetry!
 A: You can build a ladder for this. We know there's very little antimatter on Earth, or there'd be no Earth. We know the Sun isn't made of antimatter, or we'd already be wiped out. Beyond the Solar System such statements are harder to make, but can still be done. As long as there's antimatter somewhere, it'll annihilate with our portion of the universe that we know is matter, emitting photons of well-known energy. These photons can be detected. The ladder goes bigger and bigger and bigger until the only remaining (semi-viable) possibility is that entire galaxy clusters are made of antimatter. This itself is getting less and less likely. See this paper: https://arxiv.org/abs/1204.4186, the section on a patchwork universe.
That leaves the possibility that perhaps somewhere outside the observable universe there is all the missing antimatter. However, the problem with this is that you can't prove or disprove it. By definition anything outside the observable universe cannot be observed. At that point, it's beyond the realm of science.
I suppose one could argue that baryon asymmetry isn't a problem because the antimatter is outside the observable universe, but I'd call that a copout, and certainly science should not accept that as an "explanation".
