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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?

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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:

  1. 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.
  2. 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.

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  • $\begingroup$ Isn't there a third explanation? All the mater - anti-matter events happened during the early history of the Universe. Now all galaxies of matter and of anti-matter are too far from each other for the expected collisions to happen. Then we wouldn't see any γ ray burst now and in any future because of the expansion. $\endgroup$ – dan Nov 12 '20 at 12:43
  • $\begingroup$ @dan Galaxies are not completely isolated from each other because the space between galaxies is not a prefect vacuum. The density of particles in intergalactic space is very low, but not zero. If matter and antimatter was annihilating even if only in small quantities we should still be able to detect the gamma ray “glow” that this would cause. $\endgroup$ – gandalf61 Nov 12 '20 at 13:06
  • $\begingroup$ Is it correct to add: if and only if, the boundary plan of annihilation is perpendicular to the direct optical path toward Earth? $\endgroup$ – dan Nov 12 '20 at 13:11
  • $\begingroup$ @dan Not really. Along the line of sight between us and a hypothetical antimatter galaxy there must be some region where matter and antimatter mix and annihilate. We would see the gamma rays produced from this region. $\endgroup$ – gandalf61 Nov 12 '20 at 13:33
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    $\begingroup$ Based on the present limitations to scientific knowledge, it is a plausible SPECULATION that in an infinite universe there might be distant regions which at the present time include anti-matter rather than matter as we know it. Speculations may be very useful in science fiction, but the absence of any possibility to ever observe such a remote part of the universe makes this speculation irrelevant to science. $\endgroup$ – Buzz Nov 14 '20 at 15:58

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