Good question! So there are a few theories about what happened to antimatter, but the thing to remember is that at the moment, we really don’t know for sure! There are lots of scientists working on trying to find this out.
The stuff that makes up me and you, particles like electrons and protons and so on, that’s all matter. But for each particle of matter, there is an antiparticle, apparently identical except that is has an opposite charge. For example, for the negatively charged electron, we have the positively charged anti-electron, called a positron. When a particle and its antiparticle come together they annihilate to form a lot of energy.
We think that the same amount of matter and antimatter was created at the beginning of the Universe. So one of the big questions in physics is, if this right, why are we still here at all? Surely all the matter and antimatter should have annihilated to just leave a universe of light.
There’s one theory that suggests that out in space there are whole anti-galaxies which are made up of only antimatter. Perhaps there are even anti-Earths with anti-Imascientists going on! But if the matter and antimatter galaxies collided in space there would be a huge amount of light from these annihilations, which we haven’t seen. Even if they didn’t collide though, we would still expect to see some anti-Heliums that have escaped the anti-galaxy. We have experiments, like AMS on the International Space Station, that are looking for anti-Helium nuclei. But so far, they’ve not seen anything.
Another theory is that there is some small difference between the way matter and antimatter change into new particles, and maybe this would explain why slightly more matter than antimatter survived. Experiments like the LHCb detector at CERN where matter and antimatter are created in proton collisions, are searching for evidence of this difference. They’ve had some hints so far, but nothing that can account for the disappearance of a universe worth of antimatter.