The observable universe contains almost no antimatter. Some naturally occurring radioactive isotopes decay via positron emission, and positrons and antiprotons are present in some high-energy environments, such as the particle showers produced by cosmic rays. But otherwise, matter is overwhelmingly dominant as the stuff of the natural world.
That’s a puzzle. According to the laws of physics as we understand them, particles and their antiparticles should behave identically, except for a difference in sign for discrete properties such as charge. They should have been produced in identical numbers in the first moments after the Big Bang. The particle–antiparticle pairs should all have annihilated, and the universe should have been left with no matter or antimatter at all.
It wouldn’t have taken much of a matter–antimatter imbalance in the primordial universe to produce the matter we see today. One extra proton per billion proton–antiproton pairs would have sufficed. But even that...