Science › Big Bang & cosmos
Primordial black holes as dark matter
1971· Theory
SpeculativeA serious idea with no experimental proof yet.
The idea that black holes formed in the universe's first second, not from dead stars, and that they make up some or all dark matter.
See it on the Science map →How it works
If a patch of the very young universe was dense enough, it could collapse straight into a black hole. Born this way, black holes could have almost any mass, from a mountain's to thousands of suns'. They would need no new particle: they are dark, heavy and almost never collide. One range remains open for them to be all of dark matter: asteroid masses, each packed into a space ranging from far smaller than an atom to about the size of a virus.
Why it matters
They would explain dark matter with known ingredients, gravity and black holes, plus an unusual early universe. Finding even one would open a window on the first second.
How we know
None confirmed. Microlensing surveys since the 1990s (MACHO, EROS, OGLE, Subaru) rule out many star-like and planet-like masses; gamma-ray limits rule out small ones. LIGO's 2015 black-hole merger revived interest.
Still a mystery
Do any exist? Could some of LIGO's merging black holes, or the giant black holes JWST sees surprisingly early, be primordial? The asteroid-mass window has no good test yet.
Connections (3)
▸Dark matter may be made of Primordial black holes as dark matterSpeculative
The relationship
Primordial black holes would behave just like cold dark matter: dark, heavy, slow, and gravitating without colliding. Crucially, they formed within the first second, before nuclei were cooked, so they do not count against the strict budget of ordinary matter set by hydrogen and helium abundances. Their mass would come from the radiation and particles swallowed in the collapse.
How the link was found
George Chapline proposed them as dark matter in 1975. Interest surged in 2016, when Simeon Bird and colleagues asked whether LIGO's first black holes could be dark matter.
Is it proven?
Not proven, and mostly ruled out: lensing, gamma rays, the afterglow and gravitational waves exclude them as all of dark matter across most masses, except roughly asteroid masses, where no test yet reaches.
Open questions and mysteries
Could dark matter be a mix of primordial black holes and particles? A black hole lighter than any star could make, found by lensing or gravitational waves, would be a smoking gun.
▸Stephen Hawking pioneered the idea behind Primordial black holes as dark matterSpeculative
The relationship
Hawking argued in 1971 that lumps in the early universe could collapse into black holes as light as a hundred-thousandth of a gram. Asking what such tiny black holes do led him in 1974 to find that black holes radiate, and smaller ones are hotter. So primordial black holes lighter than about a kilometer-wide asteroid would have exploded by now, setting a lower limit for dark matter.
How the link was found
Yakov Zel'dovich and Igor Novikov first considered early-universe black holes in 1966. Hawking's 1971 paper developed the idea; Bernard Carr and Hawking (1974) showed they would not swallow everything. In 1976 Don Page and Hawking used gamma rays to cap their numbers.
Is it proven?
Hawking radiation is accepted theory but unobserved. Gamma-ray telescopes (Fermi, HAWC) have seen no exploding black holes, which limits how many small primordial ones exist.
Open questions and mysteries
Could a primordial black hole explode nearby, giving the first direct test of Hawking radiation? A few theorists have even suggested one could be the hypothetical 'Planet Nine' beyond Neptune.
▸Cosmic inflation might have made Primordial black holes as dark matterSpeculative
The relationship
Simple inflation leaves ripples of about 1 part in 100,000, far too gentle to make black holes. But if the inflaton crossed a nearly flat ledge on its energy hill, it would slow down and amplify ripples on small scales, perhaps a thousandfold. After inflation, those strongly overdense patches collapse directly into black holes as soon as light can cross them.
How the link was found
Pavel Ivanov, Pavel Naselsky and Igor Novikov (1994) and Juan García-Bellido, Andrei Linde and David Wands (1996) linked inflation models to primordial black holes as dark matter.
Is it proven?
Not proven. The afterglow measures ripples only on large scales; small scales are barely constrained. Such spikes would also leave a background of gravitational waves, a target for pulsar-timing arrays.
Open questions and mysteries
The gravitational-wave hum pulsar-timing arrays reported in 2023 most likely comes from pairs of giant black holes, but an early-universe origin is not excluded. Building such a ledge needs careful tuning.