The question is that how can we make sure that our universe is baryon asymmetric? I mean, is it possible that there are antimatter domains beyond some very large scale. Yes, if that kind of domains exist, the scale must be very large. But does such constraint bring any problems such that we prefer to believe the baryon asymmetry?
So, there are several possible ways the universe could be baryon symmetric:
- A region of the universe where antimatter dominates. There is a problem with this theory, though - 30 years' worth of scientific research has calculated just how far away this type of region would have to be, and from these calculations it is considered very unlikely that any part of the observable universe, at least, would have this sort of region, and there isn't really any solid evidence for this theory, so these regions might not exist at all.
- The second possibility is that antimatter repels matter instead of attracting it gravitationally; however, this is in conflict with the theory of relativity.
So then there are two explanations for the universe being baryon asymmetric:
- An electric dipole movement, or EDM. If this was present in any fundamental particle, it would violate parity and time symmetries, therefore allowing matter and antimatter to decay at different rates. However, no EDM has been detected to date.
- There's something we're missing about the laws of the early universe.
Hope this helps!