Light passing by a black hole falls into a gravitational trap: its trajectory curves and the particle bounces along a nearly closed orbit, like a pinball. The longer this frantic dash, the more likely it is that jolts from magnetic fields transform the photon into an axion — an invisible particle that likely makes up dark matter. This possibility was predicted by Frank Wilczek.

If the high-energy light from active black holes dims more than expected, then some of it has converted into axions. Thus, cosmic pinball becomes a cheap laboratory for hunting dark matter, replacing ground-based experiments.