A pot of boiling water bubbles and churns, mixing everything inside. Take it off the heat, and the motion subsides as the water slowly cools. The Universe is like that: right after the Big Bang, it was a searing 'soup' of particles that constantly collided and transformed into one another. But as space expanded, the cosmos cooled, and particles met each other less and less often.

In a new study, physicists have shown that dark matter can be born precisely during this quiet period — not from direct impacts, but from the very act of cooling. They considered a special particle — the sterile singlet (a particle that doesn't take part in ordinary interactions, like an untouchable observer). Its connection to our world is so weak that no manual tuning of parameters is needed to produce the right amount of dark matter. The weakness of the coupling emerges naturally from the vast gap between two levels of nature: the scale of the electromagnetic force and the scale at which the strong interaction puzzle is resolved.

This particle turns out to be extremely light — thousands of times lighter than the electron. But it's precisely this 'fluff' that helps explain two long-standing mysteries in astronomy. First, why the density of matter in galactic centers doesn't spike to infinity but forms a smooth core. Second, why the Milky Way has far fewer dwarf galaxies orbiting it than simple models predict. Light dark matter smears matter over a larger volume, so small clumps simply don't form.

So the cooling pot of the cosmos, with no chef, cooks up the recipe for dark matter we need. And this same recipe also explains why the starry sky looks the way it does, and not some other way.