Could the matter-antimatter symmetry be local rather than universal? In the observable universe there is an antimatter-matter asymmetry. Are there any theories that propose that this is just a local asymmetry one fluctuation in a universe with an overall symmetry?
 A: I touched on this in an answer I wrote in the Astronomy SE. The brief answer is there's no theoretical reason why it can't be local, but it's observationally excluded:

At some level the idea that there are antimatter galaxies out here is appealing. First it can solve the baryon asymmetry problem at a stroke. It's also the case that an antimatter star would shine. From long distance, it would also be practically indistinguishable from a "normal" star.
However, there are strong reasons to believe that there are no antimatter galaxies. That's because antimatter annihilates with normal matter, which leaves experimental signatures. If any part of the Earth were made of antimatter, it would immediately vanish in a flash, so we can be sure that the Earth is mostly matter. Similarly, if the Sun were made of antimatter, we would be quickly annihilated (thanks to the antimatter solar wind radiating from the anti-Sun), so we can be sure the Sun is also mostly matter. Similar arguments allow us to conclude that the Milky Way is almost entirely matter, the Local Group is almost entirely matter, etc, all the way up to the largest structures in the sky.
If antimatter galaxies exist, they are probably outside our observable universe, at which point some will argue it's no longer science.

This dealt with antimatter galaxies, which is not exactly what you're asking about, but it's effectively the same: if the matter-antimatter asymmetry is local, then there must be regions of the universe where antimatter dominates, and in those regions there must be antimatter galaxies.
A: The Alpha Magnetic Spectrometer experiment, installed on the International Space Station, is designed to study antimatter before they have a chance to interact with the Earth's atmosphere or deflect in Magnetosphere.


If antimatter-dominated places of space existed, then the gamma rays produced in annihilation would be sufficient for instruments to detect.
Antimatter is produced in nature, everywhere where high energy particle collissions occur.
