Why is our universe predominantly made of matter instead of antimatter according to the Big Bang theory?

The reason the universe is predominantly made of matter rather than antimatter according to the Big Bang theory revolves around the phenomenon of particle-antiparticle pair production and their subsequent annihilation. During the very early stages of the universe, particles and their corresponding antiparticles were indeed created in nearly equal amounts. However, during the process of annihilation, a significant imbalance led to a slight excess of matter particles over antimatter particles.

As the universe expanded and cooled, most of the matter and antimatter would have annihilated each other when they came into contact, resulting in energy in the form of photons. Yet, due to a subtle asymmetry in the laws of physics, a small number of matter particles survived this annihilation process. This slight excess of matter is critical, as it provided the building blocks for the stars, galaxies, and ultimately life as we know it.

Thus, the presence of this small excess of matter particles after annihilation is what shaped our universe into a matter-dominated one, explaining why we observe a cosmos filled with matter instead of an equal mix, or a universe dominated by antimatter.