Dark matter — the invisible stuff contributing 80% of the mass of our universe — continues to puzzle scientists. While it was long hoped that dark matter’s origins could be explained by new fundamental particles, experimental efforts to find such particles have repeatedly come up short. And while there’s no shortage of new particles to consider and experiments to construct, this lack of definitive progress has recently fueled renewed interest in solutions of a more extreme nature.

In the 1970s, it was first proposed that black holes could form directly from the small, random density fluctuations inherent to the chaotic primordial universe. The studies from which the idea emerged — pioneered by titans of cosmology like Stephen Hawking and Yakov Zeldovich — were not actually concerned with dark matter, but in recent decades primordial black holes (PBHs) have gained prominence as a compelling explanation for the substance’s origin and microphysical identity.  “The idea is very simple,” CERN postdoctoral researcher Dr. Gabriele Franciolini tells Supercluster, as forming PBHs requires nothing “beyond the standard model” of particle physics, in contrast to many dark matter theories.

Unlike black holes formed from stars, PBHs could originate with nearly any mass, suggesting a wide selection of observational signatures. Even so, it wasn’t until the 1990s that efforts to test the PBH-as-dark-matter hypothesis began in earnest with the advent of gravitational microlensing surveys like MACHO. These surveys, which look for the bright flashes produced when a massive object’s gravitational field temporarily foregrounds a distant star, initially suggested there was an overabundance of dark objects in our galaxy, driving interest in PBHs.

Unfortunately it wasn’t long before the hype, much like an individual microlensing event, faded. “People got excited,” Dr. Franciolini explains, “But then the observations were refined, and [researchers] discovered that the abundance… was compatible with just the stellar population,” converting a potential discovery into one of the strongest constraints on the number of PBHs in the planet-to-stellar mass range.

For black holes heavier than a few hundred solar masses, low event rates and long lensing timescales make microlensing surveys ineffective. Instead, scientists search for signs of dynamical disruption within astrophysical environments, or look for hints in the gravitational waves produced by merging black holes. In the mid-2010s, the first detection of gravitational waves at the LIGO and VIRGO observatories seemed to support a primordial interpretation, again generating interest in PBHs. In a familiar cycle, however, more sophisticated models eventually did not favor this interpretation.

Instead, they are increasingly seen as an appealing target in an era of high-precision, data-intensive astronomy. “Asteroid-mass” black holes, which remain the last observationally unconstrained class of PBHs,  are particularly tantalizing. “The most interesting [model] to work on is the one that is not yet constrained, but is soon to become constrained,” Dr. Franciolini explains. These black holes (with masses comparable to that of an asteroid or small moon) would be too small to be seen in lensing surveys or gravitational wave detectors, but large enough to avoid disappearing through a process called Hawking radiation. On the other hand, asteroid-mass PBHs present a unique challenge for astronomers owing to their microscopic sizes and minimal gravitational pull. 

To get around this issue, most proposals to spot asteroid-mass PBHs leverage their hypothetical abundance: for such small objects to constitute dark matter, there would have to be a lot of them. So many, in fact, that we expect one should pass within our inner solar system every hundred years or so. These invisible visitors, mere nanometers across, would occasionally get close enough to planets to perturb their motion, enabling a search using precision measurements of the position of Mars. Less frequently, these close encounters would result in a direct collision.

For PBHs encountering white dwarfs, such coincidences would be catastrophic, triggering a supernova. Consequently, a universe awash in microscopic black holes may be ever-so-slightly brightened by the afterglow of these stellar detonations.

In part due to their tiny size and high interstellar speeds, asteroid-mass PBHs would often pass through planets and main-sequence stars unimpeded; only on rare occasions would one be slowed enough during an encounter to get completely captured. Eventually sinking to the cores of these objects, such concealed black holes would slowly engulf their hosts over millions or billions of years, reducing the number of stars in dark matter-heavy environments.

In another science-fiction-turned-reality idea, some scientists hope to use paleodetectors — also known as rocks — to advance their primordial pursuits. The idea isn’t entirely without precedent; rocks are often found to retain microscopic tracks of particles released during radioactive decays. Repeated puncturings by PBHs over Earth’s geological history may have similarly carved out micrometer-scale tubules in rocks, but these would be exceedingly rare. We’ll leave it for you to decide whether scouring granite countertops for dark matter traces seems worth the effort.

This mind-bending scope of these searches, inherent to many dark matter experiments, is unlikely to daunt seasoned researchers. “If you have experiments to search for it, you should do it,” says Dr. Franciolini, speaking more generally about the value of scientific experimentation. As for finally putting the primordial black hole question to rest, he remains cautiously optimistic. “We’re getting there,” he says, “I think it’s the right era.”