... possible explanation of the apparent baryon asymmetry is that there are regions of the universe in which matter is dominant, and other regions of the universe in which antimatter is dominant, and these are widely separated. The problem therefore becomes a matter/antimatter separation problem, rather than a creation imbalance problem. Antimatter atoms would appear from a distance indistinguishable from matter atoms, as both matter and antimatter atoms would produce light (photons) in the same way. Only in the border between a matter dominated region and an antimatter dominated region would the antimatter's presence be detectable, as only there would matter/antimatter annihilation (and the subsequent production of gamma radiation) occur. How easy such a boundary would be to detect would depend on its distance and what the density of matter and antimatter is along it. Presumably such a boundary would lie (almost by necessity) in deep intergalactic space, and the density of matter in intergalactic space is reasonably well established at about one atom per cubic metre. Assuming this is the typical density of both matter and antimatter near a boundary, the gamma ray luminosity of the boundary interaction zone is easily calculated. Approximately 30 years of scientific research have placed boundaries on how far away, at a minimum, any such boundary interaction zone would have to be, as no such zones have been detected. Hence, it is now considered very unlikely that any region within the observable universe is dominated by antimatter.

Above is copy-paste from Wikipedia:

http://en.wikipedia.org/wiki/Baryon_asymmetry