thermal freeze-out and relic abundance
Picture a crowded dance floor where partners are constantly pairing up and splitting apart. Now imagine the room slowly emptying as people leave. At some point the floor gets so sparse that two people almost never bump into each other anymore, and whoever is left simply stays as they are — the count of lone dancers freezes. The early universe did something similar with particles. In the hot, dense first moments, particles and their antiparticles were constantly being created and destroyed in balance, but as the universe expanded and thinned out, there came a moment when they could no longer find each other to annihilate. Their number froze, leaving a fixed leftover population — a relic.
More precisely, in the hot early universe a species of particle could be created from collisions of other particles and destroyed by annihilating with its antiparticle, and these two processes stayed in balance, called thermal equilibrium. As the universe expanded and cooled, two things happened: it grew too cold to keep creating the particles, and it grew too sparse for the surviving ones to find partners to annihilate with. At that crossover, the reactions effectively shut off — the particles 'froze out' — and however many were left over at that instant are still here today. That surviving amount is called the relic abundance.
This idea is the beating heart of one of the most attractive dark-matter stories. If dark matter is a heavy particle that interacts through roughly the weak force — a WIMP — then the freeze-out calculation predicts a leftover amount strikingly close to the dark matter we actually observe, a coincidence nicknamed the WIMP miracle. The same logic, applied to neutrinos, predicts a sea of relic neutrinos filling all of space, even colder and fainter than the cosmic microwave background. Freeze-out is thus a precise bridge: it turns the microscopic physics of how strongly a particle interacts into a cosmic-scale prediction of how much of it should be left over from the Big Bang.
Counterintuitively, the more weakly a particle interacts, the more of it freezes out: weak interactions stop early, leaving a large leftover. A particle interacting with roughly weak-force strength would freeze out with about the right abundance to be the dark matter — the heart of the 'WIMP miracle.'
When particles can no longer meet, their number freezes.
The 'WIMP miracle' is a suggestive coincidence, not a proof: it shows that a weakly interacting particle would naturally leave about the right amount of dark matter, but many other particles with other interaction strengths could also do the job, and none has been found.