Dark matter probably isn’t black holes, but the idea isn’t as far-fetched as it might sound. For decades, physicists have considered whether black holes formed in the first moments after the Big Bang could account for the invisible mass that holds galaxies together. While most evidence now points away from black holes as the full explanation, they haven’t been completely ruled out in every mass range.
Why Black Holes Seem Like a Good Candidate
About 27% of the universe’s total energy is dark matter. We know it’s there because galaxies rotate faster than they should based on their visible mass, and because light bends around seemingly empty regions of space. Whatever dark matter is, it doesn’t emit or absorb light, and it exerts gravitational pull. Black holes check both of those boxes.
The specific type of black hole that could explain dark matter isn’t the kind formed from dying stars. Those are too rare and too recent. Instead, the hypothesis centers on primordial black holes: objects that formed from extremely dense patches of matter in the very early universe, within the first fraction of a second after the Big Bang. These dense regions would have collapsed under their own gravity before stars or galaxies ever existed, producing black holes across a wide range of sizes, from smaller than an asteroid to thousands of times the mass of our Sun.
What Observations Have Shown So Far
The most direct way to search for dark matter black holes is gravitational microlensing. When a massive but invisible object passes between Earth and a distant star, its gravity bends the star’s light, causing a temporary brightening. The object doesn’t need to emit any light itself. Astronomers first described this technique in 1986 and have been running dedicated surveys ever since, watching millions of stars in the Large Magellanic Cloud (a nearby galaxy) for telltale flickering.
These surveys have produced clear results for certain size ranges. Objects lighter than about 10 times the mass of our Sun do not significantly contribute to the hidden mass in our galaxy’s surrounding halo. That eliminates a large chunk of the possible mass range. Heavier black holes are harder to detect this way because their microlensing events last longer, sometimes years, requiring survey programs that run for decades to catch enough of them. Early surveys like EROS-2 and MACHO, which operated in the 1990s and 2000s, simply didn’t run long enough to spot the brightest signals from the heaviest candidates.
Gravitational wave detectors have added another angle. When two black holes spiral into each other and merge, they release ripples in spacetime that instruments like LIGO and Virgo can pick up. The frequency and mass patterns of these mergers can be compared to what primordial black hole models predict. Some merger rates are consistent with primordial black holes formed during the radiation-dominated era of the early universe. One analysis found that the entire abundance of dark matter could theoretically be explained by primordial black holes with masses comparable to asteroids, if they formed during an early matter-dominated phase with specific conditions. That’s a narrow window, though, requiring a particular set of parameters to line up.
Why Most Physicists Are Skeptical
The cosmic microwave background, the faint glow of radiation left over from when the universe was about 380,000 years old, puts strong limits on how many primordial black holes can exist. Black holes heavier than our Sun would have pulled in surrounding gas in the early universe, heating it up and leaving detectable imprints on this background radiation. Studies of these imprints have constrained the fraction of dark matter that could be in the form of massive primordial black holes to well below 100%. In other words, even if some primordial black holes exist, there aren’t enough heavy ones to account for all the dark matter.
These constraints get even tighter when you factor in that primordial black holes would likely attract small halos of dark matter around themselves, boosting their gas consumption and making their signatures in the cosmic microwave background even more visible. Recent work has revisited these limits by accounting for how gas near the black holes would become ionized, which changes how efficiently they accrete material. Even with these more conservative calculations, the constraints remain significant for black holes above a few solar masses.
There’s also a practical problem. Most dark matter models need to explain not just how much dark matter exists, but how it’s distributed. Dark matter forms enormous, diffuse halos around galaxies. Black holes, by contrast, are compact point-like objects. Getting them to mimic the smooth distribution that observations demand requires very specific formation scenarios.
The Mass Windows Still Open
Not every size of primordial black hole has been ruled out. The constraints are strongest for black holes lighter than about 10 solar masses (eliminated by microlensing) and heavier than about one solar mass (strongly constrained by the cosmic microwave background). But there are gaps. Extremely light primordial black holes, in the asteroid mass range of roughly 10^17 to 10^22 grams, remain viable. These are too light to produce detectable microlensing events and too small to leave imprints on the cosmic microwave background. They sit in a sweet spot where current instruments can’t easily confirm or deny their existence.
This doesn’t mean asteroid-mass primordial black holes are the leading explanation for dark matter. The most popular candidates are still undiscovered subatomic particles, particularly a class called weakly interacting massive particles. But the primordial black hole hypothesis has the appeal of not requiring any new physics. Black holes are objects we already know exist. If the right conditions were present in the early universe, they could have formed in enormous numbers without invoking particles that have never been detected in a laboratory.
How the Question Gets Settled
The Vera C. Rubin Observatory, currently under construction in Chile, is designed to survey the entire visible sky repeatedly over a ten-year period. Two of its four core science goals are directly relevant: understanding the nature of dark matter and tracking objects that change brightness over time, including black holes. Its unprecedented combination of sky coverage and long observation timelines could detect microlensing events from primordial black holes in mass ranges that previous surveys missed.
Gravitational wave detectors are also improving. As LIGO and similar instruments become more sensitive, they’ll measure black hole mergers with greater precision across a wider mass range. If the mass distribution of merging black holes matches what primordial formation models predict rather than what stellar evolution produces, that would be a significant clue. The pattern of mergers, not just their existence, is what matters. Primordial black holes would have different spin characteristics and mass ratios than black holes born from collapsing stars.
For now, the honest answer is that black holes almost certainly aren’t all of dark matter, but they could be a small piece of it. The asteroid-mass window remains open, and closing it will require a new generation of instruments sensitive enough to probe that elusive range.