Do Neutrinos Explain Matter-Antimatter Asymmetry?
quantamagazine.org
quantamagazine.org
...And this one is no exception to that rule, is the end of that sentence.
99% of journalists using interrogative headlines are not being scrupulous, when they do so. This is especially true, if the question seems shocking, or even slightly surprising, in some way.
A semi-scientific article about neutrinos doesn't carry very much emotional investment among ordinary folk.
Intergalactic void is so empty that next galaxy could be made of anti-matter (it probably isn't) and we couldn't tell.
But not completely empty and we should be able to detect matter-antimatter annihilations.
Galaxies can have gamma halos. It might be that intergalactic void is more egalitarian when it comes to matter-antimatter and it shows.
This isn't actually true, there's enough matter in intergalactic space that if there were galaxies made of anti-matter anywhere in the visible universe the interface between regular and anti-matter would shine more powerfully than the galaxy, and have a very characteristic spectrum as well. Nothing of the sort has been observed.
Besides, there might be even less dense areas of space than intergalactic void, for example between clusters of galaxies. Also composition of the void at the boundary might vary smoothly between matter and antimatter and mostly just photons in between.
If there are areas of matter and areas of antimatter in the universe then the space between them in young universe would be dominated by energy (photons) pushing them apart and creating natural separation.
TLDR: It's not a sure thing in a strict sense, but it's very unlikely that there are anti-matter pockets in the observable universe.
That the anisotropies in the CMB are small puts sufficient limits on structure formation in the earlier universe that our observations (even at COBE scale, let alone from recent observatories) preclude your idea. AMS 02 results to date essentially preclude the wiggle-room of very large antimatter structures at large comoving distances which have "always" been spacelike separated from very large matter structures. BAO observations also put strong limits on the existence of large antimatter structures; it is very very hard to think of ways that gravitational collapse of antimatter can avoid leaving obvious traces on the Lyman-\alpha forest, without entirely overthrowing the Copernican principle.
As far as we can tell -- and there are thirty years of observations probing exactly this question -- there are zero antimatter AGNs and zero early superluminous antimatter supernovae in our sky; if they exist at all they are outside our Hubble volume, or there is a strongly Earth-focused conspiracy about the directions in which AGN relativistic jets point and the alignments on which they are filamented, which is also in tension with the Copernican principle, and that still does not explain the absorption lines on early SN emissions.
This does not preclude other Hubble volumes filled with antimatter or with annihilation products; most models of inflation lead to many many Hubble volumes. However the observations to date effectively preclude much antimatter, and essentially all large structures of antimatter, in our own Hubble volume.
There are some further arguments and an overview of many years worth of the large-antimatter-filled-regions-in-our-universe arguments in astro-ph/9707087 (Glashow et al., 1998)
It was never a stupid argument, but it received a lot of attention in the past couple of decades, and is now essentially resolved.
(There's an overview and citations of more recent discussion in part II of https://arxiv.org/abs/1204.4186v2 although the paper also discusses other ideas about "why isn't there more antimatter in our Hubble volume" and advances a pretty speculative idea of the authors' own.)
Assuming, of course, that matter and antimatter came into contact rather than being separated by a "neutral zone". If the gravitational force between matter and antimatter were repulsive -- and as far as I'm aware nobody has ever succeeded in measuring its sign -- then matter and antimatter might coexist on intergalactic scales without coming into contact.
(Even more speculative: If antimatter also repelled itself, the resulting "pressure" would have sign consistent with both the "dark matter" which results in elevated galactic rotation rates and the "dark energy" which pushes galaxies apart.)
Also, it wouldn't matter if the gravitational attraction between matter and anti-matter were negative, that would mean that instead of being pulled toward the most massive nearby collection of matter an anti-matter galaxy (or cluster) would be repulsed. But that would simply drive the anti-matter galaxy into another region filled with matter galaxies, just at a lower density. Your theory really only works if there are enormous divisions between matter and anti-matter sections across the entire visible Universe, which there's no evidence of whatsoever. Even an anti-matter galaxy hiding in a massive intergalactic void doesn't help much because even such voids are filled with low density gas and dust clouds.