It looks as if a choice was made when it could have gone either way, and we don’t know how or why:
The first part to the puzzle is recognizing that this really is an existential problem. The Universe really is made of matter and not antimatter, and this is not a problem that’s going to go away. It isn’t the case that some parts of the distant Universe are made of antimatter and the Universe is really matter-antimatter symmetric; it isn’t plausible that the matter we see is due to a random, pro-matter (and anti-antimatter) fluctuation in the early Universe; it isn’t a problem that disappears if we hypothesize an equal-and-opposite antimatter Universe as a counterpart to our own.
Whenever and wherever antimatter and matter meet in the Universe, there’s a fantastic outburst of energy due to particle-antiparticle annihilation, and we don’t see that anywhere on large scales.Ethan Siegel, “No, Physicists Still Don’t Know Why Matter (And Not Antimatter) Dominates Our Universe” at Forbes
Recently, theoretical physicist Ethan Siegel crossed our screen while making clear why, in his view, a multiverse MUST exist. If we know so little about the actual universe that we can’t answer the title question, what sense does it make to insist that there must be an infinity of universes?
Here’s a thought: Most such people probably can’t help it. Their need to discredit something is likely greater than their need to know anything.
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See also: Physicists: New Approach To Antimatter Offers Promising Results
Clue about antimatter: Does it depend on how neutrinos behave vs. antineutrinos?
Improved precision in matter-antimatter difference does not resolve mystery
Experiments on antimatter are now possible?