Matter–antimatter asymmetry

A rock, a star and your body are made of ordinary matter. Antimatter consists of corresponding particles with the same mass but opposite properties such as electric charge. A particle and its antiparticle can annihilate, converting their energy into other particles. Why did the early universe leave an excess of ordinary matter behind?

In an illustrative count, eleven matter particles and ten matching antiparticles can form ten annihilating pairs, leaving one matter particle. Those invented counts explain what an excess means; they are not the measured cosmic proportions.[1]

Atomic nuclei in stars and planets contain protons and neutrons, members of the particle family called baryons. The observable universe has an excess of baryons over their antiparticles. Which early-universe process generated that excess and allowed it to survive? Pair annihilation removes matching particles; it does not create the initial imbalance.

A conventional generating mechanism needs interactions that change baryon number—the count of baryons minus antibaryons—a difference between matter and antimatter processes, and departure from thermal equilibrium so reverse processes do not cancel the net production. One proposed route, thermal leptogenesis, introduces heavy neutrinos whose decays create an imbalance among leptons, the family including electrons and neutrinos. High-temperature electroweak processes can convert part of it into a baryon imbalance; competing reactions can also erase it. These heavy particles and this cosmic history are hypotheses.

LHCb’s 2025 study observed different decay rates for a heavy baryon and its antiparticle in a specified decay channel. This is charge–parity symmetry breaking, or CP violation. It tests one ingredient of an explanation; the known Standard Model sources of CP violation do not account for the cosmic excess.[1] [2] [3]

Known particle differences do not establish which process produced the cosmic imbalance.[1] [2]

An established generating mechanism is still missing. Antimatter experiments investigate parts of the explanation; they do not reproduce the entire early universe or demonstrate a complete cosmic history.[2] [3]

Annihilation alone does not explain why material remained to build stars and planets. The surviving excess is evidence about the universe’s early particle interactions, including processes that laboratory experiments may only test indirectly.[1] [3]

A mechanism would connect two kinds of evidence: particle interactions measured in the laboratory and the abundance of ordinary matter inferred from the universe. Leptogenesis makes the connection especially sharp because some models also explain small neutrino masses. Confirming a relevant neutrino property would strengthen particular models without reconstructing the early universe or proving that they generated its matter.[1] [3]