Black holes are not what makes up dark matter, at least not most of it. Decades of observations have progressively ruled out black holes as the primary component of dark matter across nearly every mass range. However, a small and shrinking window remains open for a specific type of black hole that could account for some fraction of the universe’s missing mass.
The idea is not unreasonable on its face. Black holes are literally dark: they emit no light, they have mass, and they exert gravitational pull. Dark matter also has mass, exerts gravity, and doesn’t interact with light. So why aren’t they the same thing?
Why the Idea Seems Plausible
Dark matter makes up roughly 27% of the universe’s total energy content, yet no one has directly detected the particle (or particles) responsible. Every attempt to catch a dark matter particle in a laboratory detector has come up empty. That ongoing mystery has kept alternative explanations alive, and black holes are one of the most intuitive alternatives. If the early universe produced enough black holes before the first stars even formed, those “primordial” black holes could theoretically be scattered throughout galaxies today, pulling on stars and gas exactly the way dark matter appears to.
Primordial black holes differ from the ones created by dying stars. Stellar black holes form when massive stars collapse at the end of their lives, a process that started hundreds of millions of years after the Big Bang. Primordial black holes, by contrast, could have formed in the first fraction of a second, when extreme density fluctuations in the rapidly expanding universe compressed pockets of matter so tightly they collapsed directly into black holes. Theoretical models suggest several formation pathways, from processes during cosmic inflation to events called first-order phase transitions in the cooling early universe.
How Microlensing Surveys Ruled Out Most Sizes
If dark matter were made of black holes drifting through galaxy halos, those objects would occasionally pass between Earth and a distant star. When that happens, the black hole’s gravity bends the star’s light, temporarily brightening it in a predictable way. This effect is called gravitational microlensing, and astronomers have been watching for it since the 1990s.
Two major surveys, MACHO and EROS, monitored millions of stars in the Large Magellanic Cloud (a small galaxy orbiting the Milky Way) for years. If black holes in the mass range of roughly half a solar mass to 30 solar masses filled our galaxy’s dark matter halo, these surveys would have seen frequent brightening events. They didn’t. The EROS-2 results, combined with earlier data, showed that compact objects in this range make up less than 7% of the halo’s mass. That single result eliminated the most straightforward version of the hypothesis: that ordinary-mass black holes, similar to those formed from dead stars, could be the dark matter.
What the Cosmic Microwave Background Rules Out
For larger black holes, a different type of evidence kicks in. Black holes with masses above about 100 times the sun’s mass would have spent the early universe pulling in surrounding gas. That accretion process releases energy, which would have heated and ionized the gas filling the young universe. That extra energy would leave a detectable imprint on the cosmic microwave background (CMB), the ancient light released about 380,000 years after the Big Bang.
Analysis of data from the Planck satellite, which mapped the CMB in extraordinary detail, found no such imprint. At the order-of-magnitude level, primordial black holes heavier than about 100 solar masses are ruled out as the dominant component of dark matter. This closes the window on the high-mass end.
Gravitational Waves Add a Mixed Signal
When the LIGO and Virgo observatories began detecting gravitational waves from merging black holes in 2015, some physicists wondered whether those mergers could be primordial black holes finding each other in the dark matter halo. One detection in particular, GW190521, involved black holes in a mass range that’s hard to explain through normal stellar evolution, making a primordial origin tempting.
Researchers reconstructed the mass distribution of black holes from the LIGO-Virgo catalog (GWTC-3) and found that the pattern could be consistent with primordial black holes if they originated from a specific type of density fluctuation in the early universe. But the picture isn’t clean. Several detected mergers involved objects lighter than about three solar masses, a range where primordial black hole merger rates would be far too low to explain the observations. So gravitational wave data neither confirms nor cleanly rules out the primordial black hole scenario. It offers partial support for some masses while creating problems at others.
The Last Open Window: Asteroid-Mass Black Holes
After ruling out most mass ranges, one gap has persisted. Primordial black holes with masses comparable to asteroids, around 10 trillion to 100 trillion kilograms (roughly 1017 to 1022 grams), have been difficult to constrain. They’re too light to cause detectable microlensing and too small to accrete enough gas to affect the CMB. For years, this window remained essentially unchecked.
That window is now narrowing. A 2025 study used Hubble Space Telescope observations of ultra-faint dwarf galaxies, tiny satellite galaxies of the Milky Way that are dominated by dark matter, to look for signs that asteroid-mass primordial black holes had altered the structure of star populations over time. In one galaxy called Triangulum II, the data excluded primordial black holes of around 1019 grams from making up all of the dark matter at a statistically significant level. Even making up 55% of the dark matter was disfavored. This doesn’t slam the window shut, but it means that even in the last refuge for the hypothesis, the evidence is turning against black holes accounting for everything.
Why Black Holes and Dark Matter Aren’t Interchangeable
Beyond the observational limits, there’s a conceptual mismatch. Dark matter behaves as a smooth, diffuse substance spread throughout galaxies and galaxy clusters. It forms enormous halos that extend well beyond the visible edges of galaxies. It’s effectively collisionless, meaning dark matter particles (or whatever dark matter is) pass through each other without interacting, which is why dark matter halos maintain their shape.
Black holes don’t behave this way. They are point-like concentrations of mass that interact gravitationally with everything nearby. They grow by swallowing matter. They merge with each other. They create dense clumps rather than smooth distributions. Research into how dark matter distributes itself around black holes has identified several types of overdensities: cusps, spikes, and other concentrated structures that form when a black hole’s gravity reshapes the surrounding dark matter. The relationship between the two is real, but it’s a relationship between distinct things, not evidence that they’re one and the same.
A universe where dark matter was made of black holes would also look different in its large-scale structure. The way galaxies cluster, the way gravitational lensing bends light across cosmic distances, and the way the CMB’s tiny temperature variations are distributed all point to dark matter being something far more uniform and weakly interacting than a population of black holes could produce.
Could Black Holes Be a Small Piece of the Puzzle?
While black holes almost certainly aren’t the answer to dark matter, they haven’t been completely eliminated as a minor contributor. Primordial black holes could make up a few percent of dark matter without conflicting with current data. Some physicists find this interesting because even a small population of primordial black holes could help explain certain astrophysical puzzles, like the surprisingly massive black holes found at the centers of galaxies in the early universe, or the existence of black holes in mass ranges that stellar evolution struggles to produce.
The mainstream view in astrophysics remains that dark matter is most likely an undiscovered particle, or possibly a family of particles, that interacts through gravity but not through the electromagnetic force. The leading candidates, such as weakly interacting massive particles and axions, remain hypothetical. Until one is detected, the question stays open in principle. But the accumulated evidence from microlensing, the CMB, gravitational waves, and dwarf galaxy observations makes it clear that black holes can’t carry the full weight of the dark matter problem.