I remember reading somewhere that one possibility in the early universe is that a tremendous amount of matter and antimatter both formed, most of it annihilated itself, and the small amount that remained became our present matter-dominated universe. From a few casual Google searches I have not been able to find this reference. It was probably some popular physics book written in the 1990s. Possibility one in the summary below would appear to correspond to this scenario, however.
The question is that of baryogenesis, which is not well understood. Here’s the background from Wikipedia:
The Dirac equation, formulated by Paul Dirac around 1928 as part of the development of relativistic quantum mechanics, predicts the existence of antiparticles along with the expected solutions for the corresponding particles. Since that time, it has been verified experimentally that every known kind of particle has a corresponding antiparticle. The CPT Theorem guarantees that a particle and its antiparticle have exactly the same mass and lifetime, and exactly opposite charge. Given this symmetry, it is puzzling that the universe does not have …