CP symmetry says that every interaction in particle physics must obey charge conjugation and parity. (If you don’t understand the above statement don’t worry, you just need to know such a symmetry exists.)
Though CP symmetry was thought to be an exact symmetry, it turns out it is violated by weak interactions. If CP was an exact symmetry, there should be just as much antimatter produced by the big bang as matter. However, as we look around us we should be easily convinced this isn’t the case. 🙂
However, since CP is violated, there should have been slightly more matter produced than antimatter. Soon after the big bang every antimatter particle then annihilated itself with a matter particle leaving only the excess of matter which is seen today. This is probably why there is no antimatter.