How is dark matter located if we can’t see it?
Dark matter is invisible, but gives itself away by moving other things. Look out of your window: are the trees moving? You can’t see the wind, but it must be blowing because leaves are twitching. Stars and galaxies are leaves in the cosmic wind. The distance between galaxies was gradually controlled after the Big Bang by the gravitational tug from dark matter. The largest clumps of dark matter still fling galaxies around today – faster than galaxies could move if pulled only by each other’s gravity.
A clump of dark matter in our Milky Way whips individual stars into fast orbits. Smaller clumps of dark matter have pulled stars away from streams that used to be orbiting together. The cosmic soup is lumpy. But the lumpiness depends on its ingredients. If dark matter feels forces other than gravity or is made of certain particles, lumps just smaller than those known would have dissolved. To learn what dark matter is, we are watching ever more sensitive leaves – faint stars and even rays of light – for the indirect flutter of the tiniest passing breeze.
Richard Massey, Institute for Computational Cosmology at Durham University