Could there be a region of the universe that is dominated by antimatter?

Question

I was reading some texts explaining that since our region of the world is overwhelmingly dominated by matter, and not by anti-particles, this suggests that in some region of the universe, there will be a region dominated by anti-matter.
Why must there exist a region dominated by anti-matter?
I could guess at an answer, but can't say it mathematically:
Is it an issue of symmetry, that is, for each matter particle, we have an antimatter one? In other words, if my universe has two protons, does it need to have two antiprotons too?

gandalf61 · Answer

If there were any regions in the observable universe where antimatter was more abundant than matter then we would be able to observe the gamma radiation produced by the annihilation of matter and antimatter at the boundaries of these regions. Even if the boundaries were in intergalactic space, the gamma radiation would still be detectable. But no such gamma radiation has been detected, so we are fairly sure that the matter is much more abundant than antimatter throughout the observable universe.
This leaves two alternatives:

There was some asymmetric mechanism in the early universe that created more matter than antimatter, so leaving a residue of matter once most of the antimatter had been annihilated. We don't know exactly what this mechanism could be, although there are some candidates.
There are regions dominated by antimatter, but we cannot see them because they are outside of the observable universe.

The Wikipedia article on baryon asymmetry discusses this in more detail.

Could there be a region of the universe that is dominated by antimatter?

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