New study constrains asteroid-mass black holes as dark matter candidates

There are two ways to think about how black holes come to be. The first is familiar enough: a massive star dies, collapses inward, and leaves behind a knot of spacetime so dense that nothing escapes it. But there is a second possibility, one that has occupied theoretical physicists for decades. In the first fraction of a second after the Big Bang, when the universe was still incomprehensibly hot and dense, pockets of matter could have collapsed on their own, creating what scientists call primordial black holes. These objects would have formed not from stellar death but from the violent birth of the cosmos itself.

For years, researchers have wondered whether primordial black holes might solve one of astronomy's deepest puzzles: the identity of dark matter. The universe appears to contain far more mass than we can actually see. Galaxies rotate too fast. Clusters hold together too tightly. Something invisible is pulling on everything, and no one knows what it is. Primordial black holes have long seemed like a plausible answer—a way to account for all that missing mass without invoking exotic particles we have never detected.

But a new study from researchers at Oakland University in Michigan and Rice University in Texas has thrown cold water on at least one version of this idea. The team focused on a specific category: asteroid-mass primordial black holes, objects weighing between 100 trillion and 100 quadrillion grams. These are not the supermassive monsters at the centers of galaxies, nor are they the stellar-mass black holes that form from dead stars. They occupy a strange middle ground—small enough to be numerous, large enough to persist for billions of years.

The key to the investigation lies in a consequence of Stephen Hawking's most unsettling discovery. In 1974, Hawking showed that black holes are not entirely black. They leak radiation, a slow hemorrhage of energy that causes them to shrink over time. For asteroid-mass black holes, this process is accelerating now. As they approach the end of their lives, they emit increasingly intense radiation, particularly in the form of gamma rays. If enough of these objects exist, their collective glow should be visible in the diffuse gamma-ray background that astronomers observe coming from all directions in space.

The challenge is that this background is crowded. Blazars, radio galaxies, cosmic rays colliding with infrared light—all of these contribute to the gamma-ray glow. Isolating the signal from evaporating primordial black holes is like trying to hear a single voice in a stadium full of noise. The Oakland and Rice researchers built a detailed model to subtract out the known sources, then developed a specialized software tool called GammaPBHPlotter to simulate the exact gamma-ray signature that asteroid-mass black holes should produce, accounting for Hawking radiation, particle decay, and the annihilation of electrons and positrons.

The results were disappointing for the theory. Black holes weighing around 100 trillion grams could make up no more than one part in ten billion of the universe's dark matter. The picture improved slightly for heavier objects, around 100 quadrillion grams, which could theoretically account for up to 6 percent of dark matter. That is still a modest contribution, but it leaves open the possibility that larger primordial black holes might play a role in solving the dark matter mystery.

The real limitation, however, is not theoretical but observational. The researchers were forced to rely on data from instruments launched in 1991 and deorbited in 2000—equipment now more than a quarter-century old. The gamma rays from evaporating primordial black holes fall into a frequency range that current telescopes struggle to observe, a gap that astronomers call the MeV gap. Two new missions, AMEGO-X and e-ASTROGAM, have been designed specifically to fill this gap, but neither has yet been approved by a major space agency. Both remain in the planning and testing phase, waiting for funding and institutional backing. If they eventually fly, they could finally determine whether primordial black holes of any mass contribute meaningfully to dark matter, settling a debate that has occupied cosmology for decades.