‘Imagine watching a game of billiards in which the red balls are visible but the white isn’t. Suddenly you see the red ball – an electron – move across the baize. By plotting the red ball’s path, you might be able to backtrack, as it were, the path of the invisible white ball – the WIMP – that struck it. And from this, you might be able to learn more about the direction, mass and qualities of that white ball. We’re looking to do this enough times, and with enough precision, to provide the signature of a dark-matter halo.’
