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Primordial black holes (PBHs) are hypothetical black holes that may have formed in the universe’s earliest moments, rather than from the later collapse of stars. One proposed route is the gravitational collapse of an unusually dense region in the hot early universe. No PBH has been definitively confirmed, and the conditions that could create them depend on the model of the early universe.
What is a primordial black hole?
“Primordial” means belonging to the universe’s earliest stages. NASA describes PBHs as objects theorized to have formed in the first second after the Big Bang, before stars existed. They are not discovered objects: scientists have not found definitive proof that PBHs exist. NASA’s overview of black-hole types discusses them as a theoretical possibility.
Like other black holes, a PBH would be identified by its gravitational effects, not by light it emits. The name describes a proposed origin, not a confirmed class of observed black holes.
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Collapse of an unusually dense region
In a broad formation picture, a patch of the hot early universe that was denser than its surroundings could have collapsed under gravity and formed a black hole. NASA presents this as one possible route in its Roman mission explainer.
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That does not mean every density fluctuation would become a black hole. Whether a region collapses, and what mass the resulting PBH would have, depends on the size and strength of the perturbation and on the early-universe model. The 2021 Annual Review of Nuclear and Particle Science review treats PBH formation and abundance as model-dependent; it does not establish one universal collapse threshold.
What masses could they have, and could they still exist?
NASA gives an illustrative theoretical range extending from roughly 100,000 times less massive than a paperclip to 100,000 times the Sun’s mass. This describes the breadth of proposed possibilities, not a measured PBH population or the prediction of one specific model. NASA’s black-hole overview provides that comparison.
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In the standard Hawking-radiation picture, black holes lose mass over time, with smaller ones evaporating faster. The 2021 Annual Review says a PBH with an initial mass below approximately 5 × 1014 grams would have evaporated within the present age of the universe under the standard assumptions it reviews. This is an approximate survival threshold in that treatment, not a detection limit. The review also discusses constraints from evaporation products and other observations.
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Because a PBH would not necessarily emit detectable light, researchers look for effects it could have on nearby matter, light, or other compact objects. Different methods probe different mass ranges and rely on different assumptions.
Microlensing
If a compact object passes between Earth and a distant star with a close alignment, its gravity can bend and focus the star’s light. This temporary brightening is called microlensing. NASA describes how the Roman Space Telescope could search for such events, including possible isolated objects around Earth mass. Those objects would be candidates, not confirmed PBHs. NASA’s Roman mission explainer outlines the approach.
Gravitational waves
When compact objects orbit and merge, they can produce gravitational waves. Searches for binaries in mass ranges where ordinary stellar black holes are not expected to dominate can therefore constrain possible PBH populations. The LIGO summaries below report no detections in the specific analyses described; their limits apply to particular mass ranges and formation assumptions, not to all PBHs.
- LIGO’s O3a subsolar-mass search summary (2023) reports an upper limit below 5% on PBH abundance for the analysis described. That limit is specific to the search and is not a universal bound across masses or models.
- LIGO’s O4a planetary-mass search summary (2026) reports constraints below a dark-matter fraction of unity for PBHs in the 10−6 to 10−4 solar-mass interval under specified formation assumptions. This is a result for that interval and those assumptions, not a general limit on PBH abundance.
Other indirect constraints
Researchers also examine possible effects from evaporation products, gravitational lensing, dynamics, accretion, and large-scale structure. Each method tests different masses and depends on astrophysical or cosmological assumptions, so the results cannot be combined into one simple, model-independent abundance limit. The 2021 Annual Review surveys these constraint channels.
Could primordial black holes be dark matter?
PBHs are one proposed dark-matter candidate, but the cited evidence does not establish that they make up all dark matter—or determine one fraction that applies to every possible PBH population. Observational constraints vary with mass, the assumed mass distribution, formation scenario, and search method. The Annual Review and the mass-specific O3a and O4a LIGO summaries illustrate why a limit from one search should not be generalized to all PBHs.
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Confirming Earth-mass candidates would matter well beyond black-hole astronomy. As astronomer Kailash Sahu of the Space Telescope Science Institute put it, “It would affect everything from galaxy formation to the universe’s dark matter content to cosmic history.” The remark concerns the significance of confirming those candidates, not a claim that they have already been confirmed. NASA attributes the statement to Sahu.
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