Final answer:
The Big Bang produced slightly more matter than antimatter due to a small asymmetry in the basic forces of particle physics. This allowed the remaining matter to form the stars and galaxies in our universe. If the universe were completely symmetric, there would be less matter for us and the universe as we know it.
Step-by-step explanation:
The assumption that the Big Bang produced equal parts matter and antimatter is a result of the small asymmetry in the basic forces of particle physics. While particle physics seems symmetric in matter and antimatter overall, the decay of certain particles, like neutral K-mesons, has been observed to preferentially create more matter than antimatter. This small asymmetry resulted in slightly more matter than antimatter in the early universe. If there was only one part in 10⁹ more matter, the rest would annihilate pair for pair, leaving nearly pure matter to form the stars and galaxies we see today.
This small asymmetry in the Big Bang produced a very real and important asymmetry in nature, which led to the formation of the vast number of stars and galaxies observed today. Most physicists are impressed by how small this asymmetry is, as they recognize that if the universe were completely symmetric, the mutual annihilation would be more complete, leaving far less matter to form us and the universe we know.