How hot is dark matter?

Competing theories suggest that dark matter may be cold, warm or hot. The temperature refers to particles’ speed. Cold dark matter particles sit still, like water molecules frozen in ice; hot dark matter particles zip about like molecules in steam, spreading into their surroundings. The speed of dark matter particles mattered when they emerged from the Big Bang. Set loose in computer simulations, cold dark matter particles don’t move. Gravity pulls them together, creating lumps in the primordial soup. Hot dark matter particles whiz around near the speed of light, stirring it up. Fortunately for us, most real dark matter was tepid. It created lumps that eventually grew into habitable galaxies like the Milky Way. Confusingly, the theories’ names have nothing to do with the present speed of dark matter particles. Our universe expanded so rapidly that the initial motion of any dark matter particles became irrelevant.

Then particles fell into galaxies, picking up speed. Dark matter particles in the Milky Way now move 20 kilometres (12.4 miles) every second. That’s twice as fast as hydrogen on the surface of the Sun. Richard Massey, Royal Society research fellow at the Institute for Computational Cosmology at Durham University