The cosmic imbalance
According to the Standard Model of particle physics, the Big Bang should have produced matter and its mirror image, antimatter, in exactly equal amounts. For every electron, there should have been an antielectron (a positron); for every quark, an antiquark. The problem is that when a particle and its antiparticle meet, they annihilate, converting their mass into pure energy. Had the early universe been perfectly symmetrical, this mutual destruction would have left behind a cosmos filled with nothing but leftover radiation—no galaxies, no stars, no planets, and no people.
Our existence is evidence this perfect annihilation did not happen. The universe is made overwhelmingly of matter. Observational data from the cosmic microwave background—the faint afterglow of the Big Bang—allows cosmologists to estimate the extent of this imbalance. The baryon-to-photon ratio is approximately 6 x 10⁻¹⁰. This tiny number means that for every billion particle-antiparticle pairs that annihilated, a single matter particle was left over. That lonely survivor, repeated across the cosmos, formed everything we can see and touch. The fundamental question is: what caused this life-giving asymmetry?
Hunting for an answer at CERN
In 1967, the physicist Andrei Sakharov proposed three conditions necessary for a universe to generate more matter than antimatter from a symmetric beginning. First, there must be a process that violates baryon number, meaning a way to change the net count of baryons (like protons and neutrons). Second, fundamental laws must violate both Charge-conjugation symmetry (C) and the combination of Charge and Parity symmetry (CP), meaning physics must not treat particles and antiparticles identically. Third, these processes must occur out of thermal equilibrium. If these conditions were met in the early universe, a small surplus of matter could have been created and preserved.
The physical location for this point of interest is CERN, the European Organization for Nuclear Research, where scientists are actively testing these principles. Experiments at the Large Hadron Collider (LHC) are designed to find the source of the matter-antimatter imbalance. The LHCb (Large Hadron Collider beauty) experiment, for instance, studies the decay of particles containing beauty quarks. It has observed CP violation in the decay of B mesons and, for the first time, in baryons—the family of particles that includes protons and neutrons. The amount of CP violation measured so far within the Standard Model is insufficient to explain the vast cosmic asymmetry we observe.
Other experiments at CERN's Antiproton Decelerator facility create antimatter to study its properties directly. The ALPHA (Antihydrogen Laser Physics Apparatus) experiment builds and traps antihydrogen atoms—the antimatter equivalent of hydrogen. By precisely measuring their spectral lines and other properties, scientists look for any tiny deviation from their matter counterparts. So far, they appear identical. The AEgIS (Antimatter Experiment: Gravity, Interferometry, Spectroscopy) experiment intends to measure the effect of Earth's gravity on antihydrogen, answering the question of whether antimatter falls up or down. These experiments probe the universe's most fundamental symmetries, looking for a crack in our understanding that could finally explain why we exist at all.