If there were a region of antimatter somewhere we would expect to see annihilation between matter and antimatter happening wherever the boundary between those regions is. It releases a lot of energy, but we don't see that happening.

Still why is there an asymmetry. That is what we really want to find out. Especially because the universe was at one point very small and compact. It should have annihilated then.

To put it simply, we understand that more matter was created than antimatter because we exist, if matter and antimatter was made in equal proportions, the two would cancel each other out and leave nothing but heat and light. And to answer you “uneven distribution” hypotheses; that makes sense now, but when the Big Bang happened there wasn’t enough space to have any kind of distribution, let alone an uneven one. Both matter and antimatter, theoretically, came from the same singularity so there had to be an unequal amount in order for matter to survive

The weak force that is responsible for nuclear decay behaves slightly different between matter and antimatter something the scientists call CP violation. This discrepancy isn’t nearly enough to answer why Big bang ended with nearly all matter and next to no antimatter as far as we know.

it's an open question.

This would require that the entire observable universe be entirely within a small enough part of a "net normal matter" region that the matter appears to be uniformly distributed and there's no observable sign of the reduced matter density and the matter/antimatter interactions that would occur in the boundary regions. This violates the Copernican principle, that the appearance of the universe isn't a result of us occupying any special location within it. "We just randomly ended up in an all-matter region" isn't a very compelling or useful explanation.

The issue is not that it's physically impossible, but your proposal is inherently untestable and provides no additional predictive power, so this sort of thing would only be considered if it's a logical consequence of some other, more falsifiable prediction, or if we could actually observe some form of variation across the visible universe that could be a sign of such regions outside of it.

It seems like electrons and holes in semiconductors. As soon as you ask a hole to leave the semiconductor, this is not as likely as an electron leaving the semiconductor.

In (grand unified theories) GUTs the matter–antimatter imbalance is explained by “GUT baryogenesis”: extremely heavy GUT bosons (or scalars) in the early universe decay in a way that violates baryon number, violates CP, and happens out of thermal equilibrium, which together produces a tiny net excess of baryons over antibaryons. These bosons are also responsible for the decay of the proton into a postitron and the effects are really really tiny.

What would cause matter and antimatter to be produced in different, casually separated, places? This doesn't solve the problem, it just creates a different problem.

Space isn't empty, it's full of gas and dust. In vast intergalactic voids that gas might be very low density, lower even than 1 atom per cubic meter, but it's still present. And on large scales it adds up. 1 atom per cubic meter is still a million trillion tonnes per cubic lightyear, for example. Which means that between any region which is bulk dominated by anti-matter and one that is bulk dominated by matter there will be a vast boundary where annihilation reactions occur as the diffuse gas/anti-gas in intergalactic space comes into contact. If you do the math on this you find that such a region would be releasing a tremendous amount of energy, and it would keep releasing a tremendous amount of energy for billions upon billions of years. The bubble of annihilation reactions around any group of anti-galaxies or anti-galaxy clusters would outshine the galaxies themselves, and be extremely distinctive as well. We do not observe anything like that anywhere in the universe, which is pretty solid evidence that no bubbles of bulk anti-matter exist in the observable universe.

Additionally, there is no reason to imagine that there should just happen to be a bubble of matter the size of the entire observable universe.

More recently, we have more or less cracked the key mysteries of the matter/anti-matter imbalance. We've known for a while that the symmetry was imperfect, but we've also learned there are some decay reactions which have a bias toward producing matter instead of anti-matter. We still don't know exactly why this is the case though, and there are many more details to fill in on.

Matter and anitmatter annihilate to make energy. After the Bang, there remained a surplus of matter after all antimatter collisions had happened. That matter is the Universe, including us. QED. The defense rests.

The early universe was simply too energetic for matter as we know it to exist, so the assumption is that there was nothing but energy in the form of high energy gamma rays. Eventually due to spontaneous pair production, these photons decayed into electron/positron and proton/anti-proton pairs. So the only process by which we know energy can spontaneously form matter requires the production of an equal quantity of antimatter. The mystery is why these particles didn't immediately annihilate with their anti-particle counterparts. Either there was some sort of bias by which matter was produced at greater quantities, and we made of the left-overs from the mutual annihilation, or the matter and antimatter somehow got separated by inflation, or there is another mechanism for creating matter that we don't know about yet.

That would require anti matter to function in a way different from how is been observed in reality. There's nothing in this universe that can explain the disparity. Is it's invisible how and why is it invisible because that's a Rustles Teapot argument and has no basis to be considered valid.