FWIW, I certainly don’t check the board nearly as regularly as I did in past years (past decades?) due to life circumstances, but my DMs (with email notifs on) are always open if anyone ever wants to summon thoughts from this specific physicist.
There are a lot of directions here. I’ll sketch some of them, and I’m happy to go into more details on any.
Annihilation products. This is the one you were exploring above, namely that the interface between matter and antimatter domains would produce photons. As you expect, this is indeed well-trodden territory. No excesses in suitably redshifted photon fluxes are seen, and based on observed distributions of matter in the universe and constraints on matter densities in the intergalactic medium, current data is sufficient to exclude the presence of matter/antimatter boundaries in the observable universe.
Cosmic rays. Our little pocket of space is bombarded by cosmic rays. These come from numerous processes at solar system, galactic, and intergalactic scales. Most of these cosmic rays are boring things like electrons and protons, but antimatter can also appear. Barring new physics, this antimatter comes from collisions of ordinary cosmic rays with other regular matter in the universe or in violent processes like supernovae and gamma ray bursts. The more complicated the antimatter gets, though, the harder it is to make using mundane physics. Antiprotons? Not that hard to make. Antideuterons? Sure. Antihelium-3? Pretty rare. Antihelium-4? Very rare.
The space-based experiment AMS is designed to measure cosmic ray fluxes. Among its science goals is measuring the antimatter cosmic ray fluxes near Earth to look for anomalies. An important note, though, is that estimates for the rates of the “rare” stuff are hard to build, and different attempts at this lead to widely varying predictions. It’s just a very hard problem given available astrophysical data and our current understanding of the most violent processes and regions in the universe. Some of the AMS measurements are within the realm of “looks normal” and others are kind of weird, but nothing definitive. The first piece of potential new physics that would enter the fray is dark matter, as dark matter annihilation can lead to additional antimatter fluxes, so a lot of literature has been produced trying to tie AMS observations to dark matter theories.
There is a lot more to be said here, but I’ll pause except to note that a new balloon-based experiment called GAPS will make similar measurements soon.
Early universe dynamics. If you try to separate matter and antimatter into distinct regions, you need a mechanism to do that. There is a great corpus of cosmological data that gives insight into the dynamics of (anti)matter in the early universe. Cosmic microwave background radiation, large-scale structure (e.g., superclusters, filaments), and more have well-measured detailed patterns that match exceeding well what would be expected if the random, entropic processes in the early universe, together with the structure-forming influence of gravity, proceeded as expected. In other words, there is no evidence that anomalous large-scale dynamics were present. One specific conclusion is that, if there were antimatter domains, they would have to still be comingled with matter domains at least until a time well after recombination, and they would be quite noticeable then.
Baryon/photon ratio and abundances of light elements. These closely related quantities measurable through cosmological observations point to two key conclusions: there is a massive deficit of baryons relative to photons in the universe, indicating significant early (anti)baryon annihilation, and there was not “late”-time antimatter present to mess up the balance of primordial light elements, for which observations match expectations quite well.
More “how” than “why”: We know the Standard Model has all the qualitative mechanisms needed to take a hot soup of equal parts matter and antimatter and turn it into a soup with a slight excess of matter. However, the Standard Model cannot get the job done quantitatively.
In particular, quarks are the only particles so far known to violate charge-parity symmetry, but they don’t provide enough of this crucial ingredient to get to the level of imbalance needed in the early universe (about one part per ten billion). Many particle physics experiments are searching for new sources of charge-parity symmetry violation to fill this quantitative gap.
As to “why”: the anthropic principle could be invoked here. If an asymmetry weren’t present, we wouldn’t be here asking the question.
Matter/antimatter annihilation is very well studied in the lab, as it’s not hard to make antimatter for study. The details of any particular annihilation will depend on the particles involved and how “hard” they’re hitting each other. Using your language, it’s definitely more the “sudden upon contact” picture than any sort of fizzing sensation upon approach. If instead of individual particles you wanted to contemplate a hypothetical block of anti-material in empty space smacking into a regular matter block, then the first moments of contact would release enough energy to vaporize the two blocks entirely, halting any further macroscopic annihilation.