If a black hole spins quickly, it can hold a cloud of ultralight particles around it, so light that their mass almost vanishes. They arrange themselves in layers, like electrons in an atom, only here gravity does the job instead of electric forces. This structure stores energy, like a coiled spring.
A passing gravitational wave, a ripple from a distant cosmic collision, shakes the cloud. The particles plunge to lower "orbits" in unison, emitting a synchronized gravitational pulse — far more powerful than the one that woke them. It arrives with a noticeable delay and reveals itself through a distinctive shape. An unexpected twist: each such ejection slows the spin of the black hole: it decelerates just a little.
Clouds of such particles are a perfect candidate for dark matter, the invisible substance filling the universe. By catching this characteristic "echo," future gravitational-wave detectors (like the space-based LISA observatory) will prove dark matter is real for the first time. This idea, inspired by the work of Stephen Hawking and Kip Thorne, turns black holes into colossal laboratories.
Around a black hole with the Sun's mass, such a cloud would grow to hundreds of kilometers across, a true invisible cocoon the size of a planet.