baryon asymmetry

Hey Lucky star, I think someone beat you to this question on Facebook

This is an unsolved problem in physics, but there are two broad suggestions.

First is that there are in fact equal amounts, but they are kept apart somehow. However, since intergalactic space isn't quite empty, if there were galaxies/clusters/superclusters of antimatter in our observable universe, we would see a very recogniseable gamma-ray signal. This raises more questions than it answers, really.

The more likely one is that there are some physical processes that create a numerical imbalance in matter and antimatter. Ideas for these sorts of mechanisms are collectively called baryogenesis and leptogenesis. They're based on the violation of baryon number and lepton number, which are probably purely empirical conserved quantities (we assume they're conserved because we've never seen otherwise).

A key part of matter-antimatter asymmetry is CP violation: the weak force is known to treat particles and antiparticles slightly differently.