Is it possible to have giant sections of the universe made of antimatter? If so, what would the borders between matter and antimatter look like?
Ah, you've hit upon the famous problem of baryon asymmetry, or, to put it in English, why there appears to be more matter than anti-matter in the universe. Neither the standard model of particle physics, nor the theory of general relativity provides an obvious explanation as to why that is the case.
Antimatter galaxies, if they exist, are expected to have the same chemistry and absorption and emission spectra as normal-matter galaxies, and their astronomical objects would be observationally identical, making them difficult to distinguish. So how could we can tell them apart...
Well, antimatter and matter tend to celebrate each of their meetings with very spectacular gamma-ray fireworks. If a lone galaxy or lone galaxy cluster were made of antimatter, observing the fireworks upon the boundary would be academic (easy). Astronomers have not yet made such observations. Ergo, no visible part of our accessible light-cone appears to be made out of anti-matter.
Now a more interesting interpretation of the problem is to state that anti-matter is merely time-reversed regular matter. For instance, in the Feynmann diagram below (time arrow is left to right), we can either interpret it as an electron and an anti-electron annihilating each other, or (time-reversed) as an electron receiving a giant energy package and turning backwards in time. If you accept this interpretation, the question of where all the antimatter in the big bang went off to becomes trivial -- in the other time-direction -- as if there were two light-cones emanating from the big bang instead of one. So anti-matter aliens about 28.6 billion years
ago might be wondering where all the
matter might be hiding.
Having antimatter pockets in vast reaches of the universe would mean that on the edges there would be a lot of matter-antimatter annihilation. That annihilation would create very strong gamma radiation which would be capable of blowing away other matter and antimatter from the border; basically, the border might clean itself. If the uniform pockets were really large (say tens of billions of lightyears across), repeated mixing could take a lot of time to start. From within the pocket, though, it might seem as if the whole sky was on fire tens of billions of years ago, like a kind of background radiation.
- Matter and antimatter are straightfoward : Both can form atoms, both have gravity and can interact gravitationally.
- Antimatter can form suns just like matter, and generate photons from such suns just like matter. If a galaxy was made of antimatter, we could hardly detect this from our planet, because the light given out by such galaxy would be made of pretty standard photons just like our own galaxy.
- Matter and antimatter interaction happens via anihilation. If a particle of matter touches a particle of antimatter, both are destroyed and gamma rays are released in a large ammount.
This means that, besides the exceptional case of two galaxies, one made of matter and the other made of anti-matter, colliding in a expetacular series of anihilation explosions, we could barely detect that a distant galaxy is made out of antimatter, as we could not detect anything unusual from it.
Since the observable matter appears to lie along a plane, what if the anti-matter is concentrated along an axis perpendicular to that plane? If the concentration of anti-matter was great enough, then the anti-matter would only be detectable by the effects it has on observable matter. Also, being separated from matter would preclude annihilation. It could be that during the big bang annihilation occurred in most regions of a spherical explosion except the plane of the observable universe and some other region separated by a considerable distance. If that distance is great enough, then the anti-matter would not be observable. Just food for thought.