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There's also the possibility of a much larger universe filled with "cells" of matter and antimatter. While there would be annihilation at the boundaries, it would be very tenuous, as the radiation would push both regions apart (in the same way a drop of water survives on a hot pan by floating on a cushion of vapor).

http://en.wikipedia.org/wiki/Plasma_cosmology#Alfv.C3.A9n_an...

Not endorsing this theory, just illustrating some possibilities.

Putting aside this theory (which I believe is bogus): It's absolutely possible that huge cells exist, but the radiation pressure would have to be very high to actually push those regions apart and we'd probably still still see lot of stuff colliding. Even if the boundaries are huge and uniform, the effects should be visible. Apart from the radiation we would also observe boundary galaxies that are stripped of their gas and dust clouds. As you suggested, it might very well happen somewhere out there beyond our horizon, as we really have no idea how big the universe actually is.

There wouldn't be any real interaction between such cells, would there? From the perspective of ourselves, in our observable Universe, perhaps the detail of such other cells can't ever be measured in the same way scales smaller than the Planck distance and time can't be measured. That is, quantum effects occur at the largest scale as well as the smallest.

> There wouldn't be any real interaction between such cells, would there?

That's a difficult question to answer without a simulation. All we know is how the visible portion of the universe is structured. The matter seeding works a bit like a procedural terrain generator (where quantum fluctuations introduce a chaotic and self-similar structure) and then gravity and expansion do their thing, so we end up with a universe that is filled with a uniform web of filaments made up of galaxies. Both simulation and observation agree with this model. The primary interaction mechanism at this scale is gravity.

Now if there are pockets of antimatter in there, they'd either be separated from normal matter early on in the process (leading to huge gaps between the cells) or they would visibly communicate with their nearest neighbors when they annihilate stray matter. I don't know which would be more likely. But either scenario is observable if it happens within our field of view, and so far that doesn't seem to be the case. There might be something like this hidden in the ultra-deep field close to the edge of time, but it currently doesn't look like it.

> if there are pockets of antimatter in there, they'd [...] be separated from normal matter early on in the process (leading to huge gaps between the cells)

Thanks to inflation, I suspect that something like this might indeed be possible. Any cosmologist around who knows enough about ΛCDM to give an educated guess?

I don't understand why antimatter stars would emit more gamma rays than normal matter. The only difference is the charge right? Then mass and energy should be the same. That is why matter and anti-matter annihilate each other completely because neither has more energy/mass than the other, just opposite charge. And if your are referring to galactic collisions between matter and antimatter galaxies most galactic collisions do not result in star collisions because of the vast spaces in between stars within galaxies. But I could imagine gas clouds from the two opposing galaxies could come into contact and turn matter into gamma emissions.

How about whole galaxy cluster made of antimatter?

Isn't the void separating clusters really really empty?

And what if roughly half of the galaxy clusters are made of matter and half of antimatter? Would the annihilation radiation be detectable from the background? How do you detect if gamma rays are normal or additional if space might be filled with such matter-antimatter boundaries?

> How about whole galaxy cluster made of antimatter?

Galaxy clusters are too small a structure to be candidates for this. They still interact with their neighbors' matter, they pass through intergalactic particles all the time, and they're simply not isolated enough to do this undetected. At a larger scale, the universe is not cluster-like, it's filament-like:

http://cosmicweb.uchicago.edu/filaments.html

Those filaments are all gravitationally connected and they share the same type of matter from which they were created.

> Isn't the void separating clusters really really empty?

No, vacuum is relative. While the matter density of intergalactic space is very low, the sheer size of that space means there is a lot of stuff in it. The intergalactic space is mostly filled with protons that would be detectably annihilated if they came in contact with an antimatter galaxy. Furthermore, galaxies are colliding and spewing jets of stuff at each other all the time. There would always be annihilation going on that we could see.

> And what if roughly half of the galaxy clusters are made of matter and half of antimatter? Would the annihilation radiation be detectable from the background?

Yes, in two ways. First, if there were antimatter pockets as "small" as galaxy clusters within our observable universe, we would detect massive annihilation patterns along their boundaries from a time when the universe was much more dense than it is now. This is not the case, as the CMB is uniformly chaotic and mirrors the likely structure of the observable universe in its infancy. There are no such pocket boundaries visible in the cosmic background radiation.

Second, there would still be annihilation going on, both from anti-galaxies plowing through the intergalactic proton plasma and from galactic collisions. This would mean seeing antimatter galaxies really lighting up in the gamma spectrum and we'd also see some very long-lasting otherwise unexplained intense gamma sources in the sky. Both are not occurring within our visible horizon.

There is also the distinct possibility that antimatter pockets would, due to billions of years of annihilation events, be separated from matter pockets by huge gaps. We do not see such gaps in the distribution of matter.

Again, all this may be going on beyond our horizon, but if it does that also means antimatter regions would have to be vastly bigger than galaxy clusters.

Thank you very much for your answers. They are very informative.

> The intergalactic space is mostly filled with protons that would be detectably annihilated if they came in contact with an antimatter galaxy.

Are we sure of that? Namely are we sure that it is not equal mixture of protons and antiprotons? If that was the case matter galaxies and antimatter galaxies would look equally "hot" in gamma spectrum.

When we look at the spectrum of radiation of galaxies is it all explained just like spectrum of a star can be fully explained by laws of physics?

> This is not the case, as the CMB is uniformly chaotic and mirrors the likely structure of the observable universe in its infancy. There are no such pocket boundaries visible in the cosmic background radiation.

What if matter and antimatter were well mixed at the time that CMB comes from?

I pretty convinced about your argument that we should be observing sometimes that a collision between galaxies is bit "hotter" in gamma spectrum but how often would that occur would depends on how well matter and antimatter in the universe are separated. If the separations originated at the time that CMB comes from it might be pretty good and not apparent as a few easy to observe well defined pockets and gaps. They also might be increasingly separated as universe expands.