Why is there more matter than antimatter?

When Paul Dirac’s 1928 equation predicted antimatter, it came with unsettling symmetry: every particle has a mirror twin, and the laws of physics treat the pair almost identically. Applied to cosmology, that symmetry is a catastrophe. A Big Bang producing matter and antimatter in equal measure should end as pure light, every particle annihilating with its twin. The actual universe kept roughly one particle of matter per billion annihilations, and that leftover billionth is every galaxy, planet, and person. The bookkeeping error that saved us has no accepted explanation.

Why it matters

This is the question of why there is something to make things out of. It is also a precision test of physics itself, because the Standard Model, our otherwise magnificent account of particles, demonstrably cannot produce the observed excess. The gap between what the theory allows and what the universe did is a guaranteed signpost to undiscovered physics, which is rare and valuable: most anomalies might dissolve, but this one is anchored by our own existence.

What has been tried

In 1967 Andrei Sakharov wrote down the three conditions any explanation must meet: processes that change matter number, a violation of the mirror symmetry between matter and antimatter, and a departure from thermal equilibrium in the early universe. The second condition has been caught in the act: CP violation, a subtle preference distinguishing matter from antimatter, was discovered in kaon decays in 1964 and has since been measured in several particle families. But the measured amount falls short of what is needed by many orders of magnitude. So the hunt moved outward: experiments trap antihydrogen and compare it against hydrogen, finding no difference yet in spectra or in how it falls under gravity; others watch neutrinos for their own CP violation, with intriguing but unfinished hints; leptogenesis theories would blame the excess on heavy neutrino relatives decaying asymmetrically in the universe’s first instants.

Where the edge is

The mechanism is missing, not the framework. Physicists know exactly what an answer must accomplish and no observed process accomplishes it. The most promising open doors are the neutrino sector, where CP violation remains poorly measured, and searches for processes that change matter number, none yet seen.

What would count as an answer

A measured source of asymmetry, plugged into the early universe’s history, that yields one part in a billion from first principles. The number is known; what is missing is the physics that produces it.