Galaxies spin as if they contain unseen mass. This dark matter was discovered by Vera Rubin. According to one hypothesis, it consists of ultralight fields. When a star collapses into a black hole — an abyss from which not even light can escape (as derived by Karl Schwarzschild)—the surrounding field is ripped away and radiates outward as a wave. It's not sound, but oscillations akin to an echo of a strike.

Each black hole is like a beat on an invisible drum. Over the history of the Universe, as it expanded (a discovery by Edwin Hubble), trillions of such beats have accumulated. Their waves were stretched by redshift and merged into a constant hum. The frequencies lie within the audible range of 1–1000 Hz — from a low bass to a human voice. If we could perceive these fields, we'd hear a continuous drone, like the sound of a distant waterfall.

Future detectors, similar to those searching for gravitational waves, could pick up this background. Scientists have accounted for how star formation and stellar evolution influenced the loudness, while the role of chemical composition altered the pitch. A similar background noise has already been detected for neutrinos and gravitational waves — now it's the turn for dark matter.