You’re not alone; matter-antimatter asymmetry is one of the big open questions in physics. Most particle processes treat matter and antimatter identically, but there are a few areas where matter and antimatter have slightly different interactions. These occurrences are violations of Charge Parity symmetry aka CP Violation.
There must have been a certain amount of CP violation during the early phases of the Big Bang to explain our matter-dominated universe. But the known amounts of CP Violation are nowhere near enough to explain the asymmetry in matter and antimatter. There are some proposed mechanisms that would violate CP symmetry in sufficient quantities, but these haven’t been experimentally observed. There are ongoing searches to detect these processes, or related processes that would be possible if these existed. Neutrinoless double beta decay searches are one example of these detection efforts.
In summary, there’s a guaranteed Nobel Prize to whoever can answer your question.