Primordial Black Holes as Dark Matter Candidates

Primordial black holes (PBHs) are hypothetical compact objects formed in the early Universe, before the epoch of nucleosynthesis, through the direct collapse of extreme density fluctuations. Unlike black holes of stellar origin, PBHs could span a vast mass range—from sublunar scales up to thousands of solar masses—offering a non-particle explanation for dark matter. Their appeal arises from distinct observational signatures: they can act as microlenses, source gravitational waves through binary coalescences, alter the cosmic microwave background via accretion and heating, and seed early structures through Poisson fluctuations in their spatial distribution. Current constraints carve out narrow mass windows in which PBHs might still constitute a significant fraction of the dark matter. At the same time, proposed detection strategies using high-precision timing arrays, next-generation gravitational-wave observatories and deep microlensing surveys promise to probe these windows further. By integrating theoretical modelling of PBH formation during inflationary and phase-transition epochs with multi-messenger observations, researchers are assembling a coherent picture of how primordial black holes might account for, or at least contribute to, the cosmic dark matter budget.

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Primordial Black Holes as Dark Matter Candidates publication trend

The graph below shows the total number of articles in primordial black holes as dark matter candidates across all publications each year (not limited to Nature Index journals).

Technical terms

Primordial black hole (PBH): A black hole formed in the early Universe from collapsing density perturbations rather than stellar death.

Dark matter fraction (fPBH): The proportion of cosmic dark matter density comprised by PBHs.

Microlensing: Gravitational lensing by compact objects that momentarily magnifies background stars or quasars.

Mass function: The distribution of PBH masses, which may be narrow or extended depending on formation physics.

Poisson fluctuations: Statistical density variations arising from a discrete PBH population, which can seed early structure formation.

Gravitational-wave signature: The characteristic waveform emitted by merging PBH binaries, distinguishable from stellar black hole mergers by mass and redshift distributions.

References

1. Primordial black holes and their gravitational-wave signatures. Living Reviews in Relativity (2025).
2. Observational evidence for primordial black holes: A positivist perspective. Physics Reports (2024).
3. New Light on Dark Extended Lenses with the Roman Space Telescope. The Astrophysical Journal Letters (2024).
4. Microlensing Optical Depth and Event Rate toward the Large Magellanic Cloud Based on 20 yr of OGLE Observations. The Astrophysical Journal Supplement Series (2024).
5. Primordial Black Holes as Dark Matter: Recent Developments. Annual Review of Nuclear and Particle Science (2020).
6. Cosmic microwave background bounds on primordial black holes including dark matter halo accretion. Physical Review Research (2020).
7. Pulsar timing probes of primordial black holes and subhalos. Physical Review D (2019).
8. Clusters of Primordial Black Holes. European Physical Journal C (2019).