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Matter beats antimatter (baryogenesis)
1967· Open question
Open questionNobody knows the answer.
The unexplained process that left the early universe with about one extra particle of matter per billion matter–antimatter pairs.
See it on the Science map →How it works
Physics treats matter and antimatter almost alike, yet a tiny surplus of matter survived the early universe. In 1967 Andrei Sakharov listed what any explanation needs: a process that changes the amount of matter, a slight difference in how matter and antimatter behave, and a moment out of balance, such as a sudden transition. The Standard Model has all three, but far too weakly. The timing is unknown; most proposals place it at or before about a trillionth of a second.
Why it matters
Without the surplus there would be no atoms, stars or people, only light. It is one of the clearest signs that physics beyond the Standard Model exists.
Think of it like
Like two ledgers of a billion entries each that cancel line by line, except one side had a single extra entry. That leftover is the entire balance today.
How we know
The surplus is measured two independent ways, from Big Bang deuterium and from microwave-background ripples: both give about six protons or neutrons per ten billion photons. No antimatter galaxies exist nearby; borders where they met matter would glow in gamma rays.
Still a mystery
Which mechanism, and when? Leading candidates are heavy neutrinos decaying unevenly ('leptogenesis', proposed 1986) and a sharp Higgs switch-on aided by new particles. Neutrino experiments starting in the late 2020s and 2030s will test parts.
Connections (2)
▸Matter beats antimatter (baryogenesis) leaves the extra matter used in Quarks bind into protons and neutronsWell supported
The relationship
When quarks bound into protons and neutrons, antiquarks bound into antiprotons and antineutrons in almost equal numbers. By about a thousandth of a second these met their partners and annihilated, releasing light. Only the slight surplus from baryogenesis, about one in a billion, found no partner. Those leftovers are the protons and neutrons that later built every atom.
How the link was found
Once the 1965 discovery of the microwave background let photons be counted against atoms, the one-in-a-billion leftover became clear; Andrei Sakharov's 1967 paper set out to explain it.
Is it proven?
Annihilation itself is routine in labs. Calculations show a perfectly balanced universe would have kept roughly a billion times fewer protons than we see, so today's matter must be a prior surplus, not lucky survivors.
Open questions and mysteries
Could distant regions be made of antimatter instead? A 1998 analysis by Cohen, De Rújula and Glashow showed any such regions would have to be nearly as large as the observable universe.
▸Antimatter almost all vanished in Matter beats antimatter (baryogenesis)Open question
The relationship
Antimatter all but vanished because, as the universe cooled, every antiparticle met a particle and both turned to light; only matter's surplus was left. For that surplus to arise, antimatter must behave slightly differently from matter. Experiments find it does: some particles decay at different rates than their antiparticles. Such a difference, acting early, could tilt the balance.
How the link was found
James Cronin and Val Fitch found the first matter–antimatter difference in kaon decays in 1964 (Nobel 1980). The BaBar and Belle experiments saw it in B mesons in 2001; LHCb saw it in baryons in 2025.
Is it proven?
The differences are precisely measured. But the Standard Model's known differences are billions of times too weak to explain the surplus, so the link is real but incomplete.
Open questions and mysteries
Do neutrinos and antineutrinos differ more? Japan's T2K experiment hinted so in 2020; DUNE and Hyper-Kamiokande will test it from the late 2020s.