Whenever and wherever antimatter and matter meet in the Universe, there’s a fantastic outburst of energy due to particle-antiparticle annihilation. We actually observe this annihilation in some locations, but only around hyper-energetic sources that produce matter and antimatter in equal amounts, like around massive black holes. When the antimatter runs into matter in the Universe, it produces gamma rays of very specific frequencies, which we can then detect. The interstellar and intergalactic medium is full of material, and the complete lack of these gamma rays is a strong signal that there aren't large amounts of antimatter particles flying around anywhere, since that matter/antimatter signature would show up. In our own galaxy’s interstellar medium, the mean lifetime would be on the order of about 300 years, which is tiny compared to the age of our galaxy! This constraint tells us that, at least within the Milky Way, the amount of antimatter that’s allowed to be mixed in with the matter we observe is at most 1 part in 1,000,000,000,000,000! On larger scales — of galaxies and galaxy clusters, for example — the constraints are less stringent but still very strong. With observations spanning from just a few million light-years away to over three billion light-years distant, we’ve observed a dearth of the X-rays and gamma rays we’d expect from matter-antimatter annihilation. What we’ve seen is that even on large, cosmological scales, 99.999%+ of what exists in our Universe is definitely matter (like us) and not antimatter So somehow, even though we aren't entirely sure how, we had to have created more matter than antimatter in the Universe's past. Which is made even more confusing by the fact that the symmetry between matter and antimatter, in terms of particle physics, is even more explicit than you might think. Rosalina
