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.
as written by the authors arXiv:2605.28953
The possibility that Dark Matter (DM) is partially or totally constituted by stable Planckian remnants of light Primordial Black Holes (PBHs), suggested for instance by Loop Quantum Gravity (LQG), is investigated. Distinct phenomenological regimes are identified, including scenarios that trigger an early matter-dominated epoch. New constraints are derived on the initial PBH and final remnant abundances. We show that a significant initial abundance of PBHs lighter than $$10^3$$ kg would overproduce Planckian relics, implying that any observational evidence for such PBHs would challenge models with quasi-stable remnants. Conversely, the products of Hawking radiation from PBHs with masses between $$10^3$$ and $$10^{12}$$ kg impose that Planckian relics could only be a highly subdominant DM component. We identify a PBH mass around $$10^3$$ kg for which Hawking evaporation naturally reheats the Universe while the remnants entirely constitute the present-day DM. Such a scenario does not require fine-tuning the initial abundance of PBH of this mass, which could range from $$10^{-10}$$ to order one. These early-Universe cosmologies yield distinct observational signatures: scalar-induced gravitational waves sourced by primordial or Poisson fluctuations that are amplified by the early PBH-dominated era. Current and future observations of LIGO/Virgo/KAGRA, the Einstein Telescope and LISA, as well as probes of the effective number of relativistic degrees of freedom, can be used to probe and constrain the initial PBH abundance and the present-day abundance of Planckian relics.
Primordial black holes can weigh as much as a mountain yet be smaller than an atom.