Where is all the antimatter?
If you were to list the imperfections of the standard model - physicists' remarkably successful description of matter and its interactions - pretty high up would have to be its prediction that we don't exist.
According to the theory, matter and antimatter were created in equal amounts at the big bang. By rights, they should have annihilated each other totally in the first second or so of the universe's existence. The cosmos should be full of light and little else. And yet here we are. So too are planets, stars and galaxies; all, as far as we can see, made exclusively out of matter. Reality 1, theory 0.
There are two plausible solutions to this existential mystery. First, there might be some subtle difference in the physics of matter and antimatter that left the early universe with a surplus of matter. While theory predicts that the antimatter world is a perfect reflection of our own, experiments have already found suspicious scratches in the mirror. In 1998, CERN experiments showed that one particular exotic particle, the kaon, turned into its antiparticle slightly more often than the reverse happened, creating a tiny imbalance between the two.
