this post was submitted on 09 Jun 2024
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That's what we genuinely don't know. Based on the standard model, it should be in equal parts.
Part of me is still half-convinced that there are whole galaxy clusters of antimatter that are simply too far away from other clusters to produce any noticeable gamma rays, and the reason they didn't interact near the beginning of the universe is the same reason the whole thing didn't collapse into a super massive black hole: we don't know yet, but probably along the same lines as dark energy. A lot of it did probably interact though and that's where a lot of the CMB comes from.
I'm definitely a lay person though, I'm sure an actual physicist can tell me that's definitely not the case, I just don't know why not yet.
My personal idea/hope is that there is some other dimension of spacetime over which the big bang had directionality, emitting matter and antimatter across different poles, and that's why. That'd also mean there's an anti-universe, which is why I like the idea.
In terms of the galaxies, I believe there's enough of an observable difference that I think we would be able to detect antimatter clusters, or similar, based on emission lines but I'm not 100% on that. Huge annihilation events from colliding galaxies and clusters would have massive energy signatures unlike anything else but the frequency of this would determine how likely it would be to see the evidence.
Don't colliding galaxies mostly not actually touch? I thought there's so much space between everything it's almost entirely gravitational interactions. I'd assume almost no huge annihilation events from that, or extremely low frequency.
The stars and planets, yes, but there is a lot of very diffuse gas that does collide
Despite space being “empty” there’s still a surprising amount of stuff streaming through it. There are protons, electrons, carbon nuclei, etc constantly slamming into the Earth’s atmosphere, producing showers of radiation. These cosmic rays are the reason so many sensitive physics experiments ( like dark matter and neutrinoless double beta decay searches) are located deep underground. The earth is a good shield against these cosmic backgrounds.
Even if there was an “isolated” antimatter galaxy, it would get bombarded with matter in the form of cosmic rays. The annihilation photons are a really distinct signal that would be hard to miss. There are a number of gamma ray telescopes in space that map out sources of gammas, and they would have detected an antimatter galaxy if it existed.
If the antimatter galaxies are so far away that they’re beyond the visible universe, then there’s still the big question of why there was a segregation of matter and antimatter early on.
Oops responded to the wrong comment
There aren't. Even in "empty" space, there's about one atom per cubic meter, enough for a small amount of annihilation. We haven't detected any regions bordered by gamma ray emissions that would indicate a matter-antimatter boundary.
https://en.wikipedia.org/wiki/Baryon_asymmetry#Regions_of_the_universe_where_antimatter_dominates