Invisible Matter: When the Universe's Oven Cooled
The oven slowly cools. While it's hot, the dough rises vigorously, but once the temperature drops, the process stalls. That's roughly how dark matter was born in the 'freeze-out' model: in the hot early cosmos, particles collided and abounded, but as the universe expanded and cooled, their number froze, like baked goods set in place.