freeze-out
Think of a crowded room where everyone is constantly swapping seats — people stand up, mingle, sit somewhere new, an endless reshuffle as long as the music plays. Now slow the music and dim the lights: people stop circulating, and whoever is sitting where, stays. The final seating arrangement is 'frozen in.' The early universe did something like this with its particles. As it expanded and cooled, the frantic reactions that kept particles continually converting into one another slowed down and stopped, locking in fixed amounts. That locking-in is called freeze-out.
Here is the physics. In the hot early universe, particles collided so often that creation and destruction stayed in balance — a process and its reverse ran at equal rates, an equilibrium. But the universe was expanding, thinning out, and cooling. For any given reaction there comes a moment when the particles are too spread out, or too cold, to keep finding each other and reacting. At that point the reaction effectively switches off, and the amount of that particle left over is frozen — its 'relic abundance.' Different particles froze out at different times: neutrons relative to protons froze out around one second (setting the recipe for the light elements), and neutrinos decoupled from everything else at about the same time and still drift through the universe today as a relic background.
Freeze-out is one of the most powerful ideas in cosmology because a frozen-in abundance is a fossil — it records the conditions at the moment the freezing happened. The measured ratio of light elements, frozen out in the first minutes, confirms the temperature and density of the early universe to remarkable accuracy. The concept also underlies a leading idea about dark matter: a hypothetical heavy particle that froze out of the early plasma could naturally leave behind just about the amount of dark matter we infer today, a coincidence nicknamed the 'WIMP miracle.' Whether dark matter really is such a relic remains unconfirmed, but the freeze-out logic is sound and well tested in the cases we can check.
Around one second after the Big Bang, the reactions converting protons and neutrons into each other froze out. At that instant the universe held roughly one neutron for every seven protons. That frozen 1-to-7 ratio is precisely what set how much helium the universe could make minutes later — and the helium we measure in the oldest gas matches it almost exactly.
A frozen-in particle ratio is a fossil of the conditions at the moment it froze.
Freeze-out does not mean particles stopped moving or got cold in a literal sense; it means a particular reaction could no longer keep up with the expansion, fixing how many of that species remained.