The Big Bang created equal amounts of matter and antimatter. Every particle had its antiparticle mirror image. They should have annihilated each other completely — leaving only light. Yet here we are, made of matter. This cosmic asymmetry is one of the deepest unsolved puzzles in modern physics.
📖 Read more: Big Bang and Cosmic Inflation: How Was the Universe Born?
⚛️ The Universe That Shouldn't Exist
Basic physics and cosmology textbooks tell us the Big Bang created matter and antimatter in equal proportions. An electron and a positron. A proton and an antiproton. When they meet, they annihilate in a flash of pure energy. This means that in the first fractions of a second after the Big Bang, all matter and antimatter should have destroyed each other completely. Only photons should have remained.
And yet we are here. Not as nameless philosophers pondering existence — but because one particle of matter in a billion somehow survived. That one-in-a-billion is the universe we know. This asymmetry between matter and antimatter is one of the deepest unsolved mysteries in quantum physics.
🔬 The Three Sakharov Conditions
In 1967, Soviet physicist Andrei Sakharov published three conditions that must all be satisfied for an excess of matter (baryon asymmetry) to arise: First, there must be processes that change baryon number — reactions that don't conserve the total count of protons and neutrons. Second, the laws of physics must treat matter and antimatter differently — CP symmetry must be violated. Third, the universe must have been out of thermal equilibrium at some point, preventing any asymmetry from being erased.
💡 1 in 1,000,000,000
The number that defines our universe: for every 1,000,000,000 antiprotons that annihilated with protons, one proton survived. That one-in-a-billion is the entirety of what we see today. This ratio — 10⁻¹⁰ — must be explained by a scientific theory. The Standard Model predicts only 10⁻¹⁸ — a factor of 100 million too small. Something is missing.
📖 Read more: 100 Years of Quantum Mechanics: How It Changed the World
⚙️ Discovery #1964: CP Violation and a Nobel Prize
Even before Sakharov had formulated his theory, in 1964 James Cronin and Valentine Fitch at Princeton University discovered something startling: neutral kaons (K-mesons) did not decay in exactly the same way as their antiparticles. The laws of nature are not perfectly symmetric between matter and antimatter. The two physicists were awarded the Nobel Prize in Physics in 1980 for this discovery.
Later, the BaBar experiment (Stanford) and Belle experiment (Japan) measured systematic CP violation in B-mesons (2001) for the first time — finding sin2β ≈ 0.59–0.99, meaning B-mesons decay slightly more slowly than anti-B-mesons. However, all the CP violation described by the Standard Model's CKM matrix is still far too small to explain the matter-antimatter asymmetry of the universe.
"It's possible that the theory of how the matter-antimatter asymmetry in the universe built up may need modification. But either way we win — because there is something we don't fully understand about the universe and, therefore, something new to be found."
— Paul Harrison, Queen Mary College, Physics World 2001🔬 The Latest Evidence: CP Violation in Baryons, 2025
In July 2025, CERN's LHCb Collaboration published a landmark result in the journal Nature: the first experimental evidence of CP violation in baryons — the particles (protons, neutrons and their cousins) that make up almost all ordinary hadronic matter. The result reached 5.2σ statistical significance and is consistent with the Standard Model's CKM matrix, but remains 8 orders of magnitude smaller than the asymmetry needed to explain the universe.
The harder question remains: how much CP violation exists in the universe beyond what we've discovered? Neutrinos? Heavy quarks? New quantum interactions? The LHC's third run will accumulate data with much greater precision — and may eventually answer a question that physicists and cosmologists have been asking for 60 years.