antimatter in the Standard Model

Antimatter sounds like science fiction, but in the Standard Model it is utterly routine: every matter particle comes with a twin that is identical in mass but opposite in charge. The electron has the positron; each quark has an antiquark. Bring a particle and its antiparticle together and they annihilate, vanishing in a flash of pure energy (usually photons). It is less an exotic add-on than a built-in mirror image baked into the theory's very equations.

The reason it is built in is beautiful: when you combine quantum mechanics with special relativity, the math essentially forces antiparticles to exist — Dirac noticed this in 1928 and predicted the positron before anyone saw one. So the particle table really has a hidden second copy: for nearly every entry, an antiparticle of opposite electric charge (a few neutral particles, like the photon, are their own antiparticle). Antimatter is made routinely in accelerators and even in some hospital scans, and it obeys the same Standard Model rules as matter, almost perfectly symmetrically.

That 'almost' is one of the deepest puzzles in physics. If matter and antimatter were perfectly symmetric, the early universe should have made equal amounts that wiped each other out, leaving nothing but light — yet here we are, made of matter. The Standard Model does contain a tiny built-in difference between matter and antimatter (called CP violation), but it is far too small to explain the imbalance. Why the universe is made of matter at all is therefore listed among the things the Standard Model cannot yet explain.