While I was reading a similar question asking if other galaxy could be made of antimatter, to which the answer was: if they were, we should detect the radiation from matter interacting with antimatter on that sort of scale. But what if the missing antimatter lies outside the observable universe. Wouldn't that result in the matter dominated observable universe we live in, without any of the radiation from the antimatter lying outside it being detected?
In this model it is CP ( charge and parity) violation which gives rise to particle antiparticle asymmetry, and since the standard model of particle physics cannot account for this large violation the search is on for extending the standard model. In this case the asymmetry will appear during the time of the baryogenesis, by a microsecond from the origin,.
People propose models where the asymmetry appears as the creation of a universe and an anti universe, as for example here. This would mean that our universe was born with a fixed number of quarks, to make up the large asymmetry. A fine tuning that is not easily acceptable. If no large CP violation is found experimentally in the near and further future, maybe one will have to resort to similar models. At this time, the success of the Big Bang in modeling all known observations leads us to the search for CP violating mechanisms within the universe.
Your question really breaks down into two elements.
- "Missing" antimatter
- Antimatter galaxies being outside the observable universe.
Presumably, by "missing" you are alluding to the question as to why we observe matter and not antimatter, if for some reason we wish discount CP violation (covered by Anna).
The thing is, having antimatter outside the observable universe does not really help. We expect (barring CP violation) to have equal amounts of matter and antimatter formed in the early universe. We expect this symmetry everywhere in the form of pair production of matter-antimatter particles, say e+ and e-.
The observable universe is defined as the part of the universe from which we can have received signals such as light. Conversely, it is also the part of the universe which can have received signals from us. This means that the antimatter created here cannot have been somehow separated and moved outside the observable universe.
To conclude: The presence of an excess of anti-matter outside the observable universe would not explain the lack of it around here.
The problem with talking about things that lie outside the observable universe is that they are, by definition, not observable. There could be invisible pink unicorns out there, and you cannot prove they don't exist.
So yes, it is possible the missing antimatter lies outside the observable universe, but in itself this is not a powerful statement. You need something more: a mechanism to explain why all the antimatter got "out there". If the mechanism is well-motivated, makes testable predictions within the observable universe, and is eventually accepted as true, then it might solve the baryon asymmetry problem as well.
In the same way, we rely on GR to calculate what happens within the event horizon of black holes, and we are reasonably confident in the results even though we cannot observe inside a black hole.