Mathieu Gross

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.

Dark matter is the invisible scaffold that holds galaxies together. But how did it come to be? The standard answer: particles of dark matter once stopped colliding and "froze out" in the amount we see today. However, there's a trickier path — UFO, or ultra-fast freeze-out. Here, everything is like coffee that cools down and then heats up again as you carry it through a cold corridor.