baryogenesis
/ BAR-ee-oh-JEN-uh-sis /
Every particle of ordinary matter has an antimatter twin — an antiparticle with the opposite electric charge. When a particle meets its antiparticle, the two annihilate in a flash of pure energy. So here is a deep puzzle: the universe is built almost entirely of matter — you, the Earth, the stars are all matter, with essentially no antimatter around. If the hot early universe had made matter and antimatter in equal amounts, they should have wiped each other out completely, leaving a cosmos of nothing but light. Baryogenesis is the name for whatever process tipped the balance and let a little matter survive.
The word breaks down as 'the genesis (creation) of baryons.' Baryons are the heavy particles made of quarks — protons and neutrons, the building blocks of atomic nuclei. In the very hot early universe, particles and antiparticles were constantly created and destroyed. Baryogenesis refers to the moment in the first fraction of a second when a tiny excess of matter over antimatter was generated: for roughly every billion antiquarks, there were about a billion-and-one quarks. When everything annihilated, that one-in-a-billion leftover became all the matter in the universe today, and the annihilated billion pairs became the photons of the cosmic microwave background.
The puzzle is genuinely unsolved at the level of fundamental physics. In 1967 the physicist Andrei Sakharov spelled out three conditions any successful baryogenesis must satisfy — there must be processes that change the net number of baryons, a built-in preference for matter over antimatter (a symmetry violation), and a departure from perfect equilibrium. The known laws of particle physics (the Standard Model) do satisfy these conditions in principle, but the effect they produce is far too small to account for the asymmetry we observe. So while we are confident that some asymmetry-generating process happened, exactly which one — and what new physics it requires — remains one of the major open questions in cosmology and particle physics.
The size of the leftover is read straight off the sky. For every surviving baryon in the universe there are roughly 1.6 billion photons in the cosmic microwave background — the ashes of all the matter-antimatter pairs that annihilated. That ratio tells us the early imbalance was only about one extra quark per billion antiquarks. Everything solid you have ever touched is that one-in-a-billion survivor.
We exist because matter outnumbered antimatter by about one part in a billion.
Baryogenesis names the problem, not a settled solution. The Standard Model can violate matter-antimatter symmetry, but far too weakly to explain the observed excess, so this remains genuinely unsolved and points to physics beyond what we know.