Why is there more matter than antimatter?
The Big Bang should have made equal amounts of both, and they should have annihilated. Yet here we are.
The question
Every particle of matter has an antiparticle: the same mass, the opposite charge. When a particle meets its antiparticle, both disappear in a flash of energy. And whenever energy turns into matter, in accelerators or in the early universe, particles and antiparticles are born in equal pairs.
So the Big Bang should have produced exactly as much antimatter as matter. As the universe cooled, they should have annihilated almost completely, leaving a universe filled with light and nothing else. Instead there are stars, planets and us. Somewhere in the first moments, matter won. How?
What we know
- Antimatter is real and well studied. Paul Dirac predicted it in 1928, and the positron, the antiparticle of the electron, was found in 1932. Today antihydrogen atoms are made and held in traps at CERN; in 2023 the ALPHA experiment showed that they fall down under gravity, like ordinary matter.
- The observed universe is made of matter. If there were whole regions of antimatter, we would see gamma rays from the boundaries where it meets matter. No such signal has been found up to the scale of the observable universe.
- The imbalance was tiny. From the light left over from the Big Bang, cosmologists calculate that for about every billion antiparticles there were a billion and one particles. Everything we see is made of that one extra part in a billion.
- In 1967 Andrei Sakharov listed three conditions a universe needs to produce this imbalance: processes that change the number of particles of matter, a difference in how matter and antimatter behave (so-called CP violation), and a period when the universe was out of equilibrium.
- CP violation exists. It was discovered in 1964 in the decays of kaons and has since been seen in other particles; in 2025 the LHCb experiment observed it in baryons, the family protons belong to. But in the Standard Model of physics it is billions of times too weak to explain the matter we see.
Main hypotheses
Leptogenesis
Neutrinos may hold the key. If very heavy relatives of the neutrino existed in the early universe, their decays could first create an imbalance among leptons (the electron's family), which known processes would then turn into an excess of matter. The idea also explains why ordinary neutrinos are so light. Experiments are now checking whether neutrinos and antineutrinos behave differently; the T2K experiment in Japan has reported a first hint.
Electroweak baryogenesis
The imbalance could have formed when the Higgs field "switched on", a fraction of a second after the Big Bang, if this happened abruptly, like water boiling in bubbles. With the Higgs mass measured in 2012, the Standard Model predicts a smooth transition instead, so this route needs new particles, which the Large Hadron Collider hasn't found so far.
Other mechanisms
There are many other proposals, from decays of hypothetical superheavy particles to processes tied to cosmic inflation. Some link the matter excess to dark matter, which is about five times more abundant than ordinary matter, a ratio that may not be a coincidence.
Where understanding ends
We know that the imbalance exists, how big it was, and what conditions were needed for it. What we don't know is which process actually created it. The only mechanisms we have measured in the laboratory are far too weak. Every working explanation requires physics that hasn't been observed.
The difficulty is that the key events happened at energies and times that are out of reach. The heavy particles of leptogenesis may be so massive that no collider will ever produce them. The theory could then only be tested indirectly: through the properties of ordinary neutrinos, which would make it plausible but not proven.
This is one of the clearest proofs that the Standard Model of physics is incomplete. It describes almost every experiment perfectly, but it can't explain why there is anything in the universe to experiment on.
What would settle it
- A confident measurement of CP violation in neutrinos. The DUNE experiment in the US and Hyper-Kamiokande in Japan, due to start in the coming years, are built for this.
- Observing neutrinoless double beta decay, a rare process that would show that the neutrino is its own antiparticle, a key ingredient of leptogenesis.
- Discovering new particles or a new source of CP violation strong enough to produce the observed imbalance, and a calculation that gives the correct number.
Sources
- Christenson, J. H., Cronin, J. W., Fitch, V. L., & Turlay, R. (1964). Evidence for the 2π decay of the K₂⁰ meson. Physical Review Letters, 13(4), 138–140.
- Sakharov, A. D. (1967; reprinted 1991). Violation of CP invariance, C asymmetry, and baryon asymmetry of the universe. Soviet Physics Uspekhi, 34(5), 392–393.
- Fukugita, M., & Yanagida, T. (1986). Baryogenesis without grand unification. Physics Letters B, 174(1), 45–47.
- Cohen, A. G., De Rújula, A., & Glashow, S. L. (1998). A matter–antimatter universe? The Astrophysical Journal, 495(2), 539–549.
- Canetti, L., Drewes, M., & Shaposhnikov, M. (2012). Matter and antimatter in the universe. New Journal of Physics, 14(9), 095012.
- Planck Collaboration (2020). Planck 2018 results. VI. Cosmological parameters. Astronomy & Astrophysics, 641, A6.
- T2K Collaboration (2020). Constraint on the matter–antimatter symmetry-violating phase in neutrino oscillations. Nature, 580(7803), 339–344.
- Anderson, E. K., et al. (ALPHA Collaboration) (2023). Observation of the effect of gravity on the motion of antimatter. Nature, 621(7980), 716–722.
- LHCb Collaboration (2025). Observation of charge–parity symmetry breaking in baryon decays. Nature, 643(8074), 1223–1228.