The universe is filled with invisible dark matter. Just as a drop of water, as it evaporates, leaves behind a tiny salt crystal, so too can a black hole evaporate, leaving behind a minuscule stable remnant. This idea was developed by Stephen Hawking and Jacob Bekenstein.
New research examined whether such remnants of primordial holes, born right after the Big Bang, could have become dark matter. It turned out that if light holes were too abundant, their 'salt grains' overproduced — the cosmos would be oversaturated. But holes with a mass of around a thousand kilograms (like a small car) evaporate just in time for our era, heating the universe along the way, and their remnants perfectly match today's dark matter.
While these holes were evaporating, they amplified random density fluctuations, generating a powerful background of gravitational waves — ripples in spacetime. Ground-based and space detectors, which Kip Thorne helped develop, may pick up this echo. If the signal is confirmed, we’ll know that dark matter is a handful of 'salt' from ancient black holes.
Primordial black holes can weigh as much as a mountain yet be smaller than an atom.