In cosmology, primordial black holes (PBHs) are hypothetical black holes that formed soon after the Big Bang. In the inflationary era and early radiation-dominated universe, extremely dense pockets of subatomic matter may have been tightly packed to the point of gravitational collapse, creating primordial black holes without the supernova compression typically needed to make black holes today. Because the creation of primordial black holes would pre-date the first stars, they are not limited to the narrow mass range of stellar black holes.
In 1966, Yakov Zeldovich and Igor Novikov first proposed the existence of such black holes, while the first in-depth study was conducted by Stephen Hawking in 1971. However, their existence remains hypothetical. In September 2022, primordial black holes were proposed by some researchers to explain the Little red dots, unexpectedly large galaxies formed very early in cosmological time as observed by the James Webb Space Telescope (JWST).
PBHs have long been considered possibly important if not nearly exclusive components of dark matter, the latter perspective having been strengthened by both LIGO/Virgo interferometer gravitational wave and JWST observations. Early constraints on PBHs as dark matter usually assumed most black holes would have similar or identical ("monochromatic") mass, which was disproven by LIGO/Virgo results, and further suggestions that the actual black hole mass distribution is broadly platykurtic were evident from JWST observations of early large galaxies. Recent analyses agree, suggesting a broad mass distribution with a mode around one solar mass. See Figure 39.
Many PBHs may have the mass of an asteroid but the size of a hydrogen atom and be travelling at enormous speeds, with one likely being within the Solar System at any given time. Most likely, such PBHs would pass right through a star "like a bullet", without any significant effects on the star. However, the ones traveling slowly would have a chance of being captured by the star. Stephen Hawking proposed that the Sun may harbor such a PBH.
History Depending on the model, primordial black holes could have initial masses ranging from (the so-called Planck relics) to more than thousands of solar masses. However, primordial black holes originally having masses lower than would not have survived to the present due to Hawking radiation, which causes complete evaporation in a time much shorter than the age of the Universe. Primordial black holes form before the nucleosynthesis responsible for observed baryonic matter, and as such are plausible dark matter candidates. Primordial black holes are also good candidates for being the seeds of the supermassive black holes at the center of massive galaxies, as well as of intermediate-mass black holes.
Primordial black holes belong to the class of massive compact halo objects (MACHOs). They are naturally a good dark matter candidate: they are (nearly) collision-less and stable (if sufficiently massive), they have non-relativistic velocities, and they form very early in the history of the Universe (typically less than one second after the Big Bang). Nevertheless, critics maintain that tight limits on their abundance have been set up from various astrophysical and cosmological observations, which would exclude that they contribute significantly to dark matter over most of the plausible mass range. However, new research has provided for the possibility again, whereby these black holes would sit in clusters that disperse at later times, with most black holes forming in the QCD matter epoch at around 1 or 30 solar masses.
In March 2016, one month after the announcement of the detection by Advanced LIGO/VIRGO of gravitational waves emitted by the merging of two 30-solar-mass black holes (about ), three groups of researchers proposed independently that the detected black holes had a primordial origin. Two of the groups found that the merging rates inferred by LIGO are consistent with a scenario in which all the dark matter is made of primordial black holes, if a non-negligible fraction of them are somehow clustered within halos such as faint dwarf galaxies or globular clusters, as expected by the standard theory of cosmic structure formation. The third group claimed that these merging rates are incompatible with an all-dark-matter scenario and that primordial black holes could contribute only less than one percent of the total dark matter. The unexpected large mass of the black holes detected by LIGO has strongly revived interest in primordial black holes with masses in the range of 1 to 100 solar masses. It is still debated whether this range is excluded or not by other observations, such as the absence of micro-lensing of stars, the cosmic microwave background anisotropies, the size of faint dwarf galaxies, and the absence of correlation between X-ray and radio sources toward the galactic center.
In May 2016, Alexander Kashlinsky suggested that the observed spatial correlations in the unresolved gamma-ray and X-ray background radiations could be due to primordial black holes with similar masses, if their abundance is comparable to that of dark matter.
In August 2019, a study was published opening up the possibility of making up all dark matter with asteroid-mass primordial black holes (3.5 × 10−17 – 4 × 10−12 solar masses, or 7 × 1013 – 8 × 1018 kg).
In September 2019, a report by James Unwin and Jakub Scholtz proposed the possibility of a primordial black hole (PBH) with mass 5–15 (Earth masses), about the diameter of a tennis ball, existing in the extended Kuiper Belt to explain the orbital anomalies that are theorized to be the result of a 9th planet in the solar system.
In October 2019, Derek Inman and Yacine Ali-Haïmoud published an article in which they discovered that the nonlinear velocities that arise from the structure formation are too small to significantly affect the constraints that arise from CMB anisotropies.
In September 2021, the NANOGrav collaboration announced that they had found a low-frequency signal that could be attributed to gravitational waves and potentially could be associated with PBHs.
In September 2022, primordial black holes were used to explain the unexpected very large early (high-redshift) galaxies discovered by the James Webb Space Telescope.
On 26 November 2023, evidence, for the first time, of an overmassive black hole galaxy (O.B.G.), the result of "heavy black hole seed formation from direct collapse", an alternative way of producing a black hole other than the collapse of a dead star, was reported. This discovery was found in studies of UHZ1, a very early galaxy containing a quasar, by the Chandra X-ray Observatory and James Webb Space Telescope.
In 2024, a review by Bernard Carr and colleagues concluded that PBHs formed in the quantum chromodynamics (QCD) epoch prior to 10–5 seconds after the Big Bang, resulting in a broadly platykurtic mass distribution today, "with a number of distinct bumps, the most prominent one being at around one solar mass."
In November 2025, the LIGO/Virgo/KAGRA collaboration reported an unusual gravitational wave from a black hole merger with source masses small enough to suggest evidence for a sub-solar-mass black hole population such as primordial black holes formed in the early universe. As there have been no gravitational waves detected at z>1 (>6 Gya), and the sensitivity to lower-mass collisions falls off with distance, we are not currently able to detect collisions in the earliest half of the age of the universe.
In late 2025, observations of high-redshift "little red dots" (LRDs) and early galaxies by the JWST provided constraints favoring the existence of massive black hole seeds such as from PBHs over standard stellar remnant models. In August 2025, Juodžbalis et al. reported a direct dynamical mass measurement of a black hole in a strongly lensed LRD at redshift z=7.04, finding a central mass of about 50 million solar masses, outweighing its galaxy's stars. The authors describe it as consistent with formation in the early universe. Subsequently, Tripodi et al. identified an active galactic nucleus in the LRD "CANUCS-LRD-z8.6" at about z=8.6, containing a roughly 108 solar mass black hole that had grown rapidly before its host galaxy established significant stellar mass, consistent with extremely early formation or direct collapse. Concurrently, Chavez Ortiz et al. presented evidence for an active ~107.2 solar mass black hole in the galaxy GHZ2 at z=12.34, less than 400 million years after the Big Bang — the most distant black hole identified to date — further challenging standard accretion limits and supporting rapid formation mechanisms.
Formation
Primordial black holes could have formed in the very early Universe (less than one second after the Big Bang) during the inflationary era, or in the very early radiation-dominated era. The essential ingredient for the formation of a primordial black hole is a fluctuation in the density of the Universe, inducing its gravitational collapse. One typically requires density contrasts (where is the density of the Universe) to form a black hole.
Production mechanisms There are several mechanisms able to produce such inhomogeneities in the context of cosmic inflation (in hybrid inflation models). Some examples include:
Axion inflation
Axion inflation is a theoretical model in which the axion acts as an inflaton field. Because of the time period it is created at, the field is oscillating at its minimal potential energy. These oscillations are responsible for the energy density fluctuations in the early universe. For a review of production mechanisms arising from various models of inflation, see Ref.
Reheating
Reheating is the transitory process between the inflationary and the hot, dense, radiation-dominated period. During this time the inflaton field decays into other particles. These particles begin to interact in order to reach thermal equilibrium. However, if this process is incomplete it creates density fluctuations, and if these are big enough they could be responsible for the formation of PBHs.
Cosmological phase transitions
Cosmological phase transitions may cause inhomogeneities in different ways depending on the specific details of each transition. For example, one mechanism is concerned with the collapse of overdense regions that arise from these phase transitions, while another mechanism involves highly energetic particles that are produced in these phase transitions and then go through gravitational collapse forming PBHs.
Fine-tuning issues
The primary theoretical challenge to PBHs is the physical mechanism of their formation. Standard models of cosmic inflation, known as "slow-roll inflation," generate density fluctuations that are far too small to trigger primordial collapse. Consequently, producing the required abundance of PBHs to explain a substantial fraction of dark matter typically necessitates "exotic" inflation models, often featuring inflection points, bumps, or plateaus in the inflaton potential, which can amplify fluctuations by orders of magnitude. Critics argue that these models require significant fine-tuning, as the resulting PBH abundance is exponentially sensitive to the amplitude of these fluctuations, meaning that a slight deviation in parameters results in either a negligible amount of PBHs or a universe dominated entirely by them. However, proponents contend that as the natural parameter space for alternatives such weakly interacting massive particles is increasingly excluded by null results from all detection experiments, particle dark matter theories now require comparable levels of fine-tuning. Furthermore, they argue that the mass structures predicted by such exotic inflation models provide a unified explanation for observational anomalies seen by LIGO and JWST that particle models do not address.