A major question that physicists often ask is why there
is so much more matter than antimatter in the universe.
This is known as the Baryon Asymmetry problem. The
expected outcome of the big bang would be to create
equal amounts of matter and antimatter that will destroy
each other and leave behind a universe consisting only
of radiation. Obviously, this was not the case.
The most prominent explanation is that the process
involved with the big bang somehow favored the creation
of matter over antimatter. There are other less supported
explanations such as the theory that there is another part
of our universe that consists mostly of antimatter.