Figure 1. Shown here is the remarkable unifying power of the natural, broad primordial black hole mass function which we proposed in Carr et al., Phys. Rep. 1054, 1–68 (2024) [41] (black dashed curve). A single physically motivated spectrum traverses the mass–abundance regions associated with a remarkable variety of reported observational indications (saturated coloured bands), and does so over numerous orders of magnitude in mass. Although the figure is not, of itself, a statistical demonstration, so extensive a concordance can scarcely fail to impress; to my mind, it strongly fortifies the conviction that primordial black holes do indeed inhabit our Universe.

My research is concerned with every aspect of primordial black holes (PBHs)—black holes formed in the early Universe—including the manner of their formation, their phenomenology and their quantum structure. The modern cosmological theory of these objects was developed in seminal work by Stephen Hawking and my collaborator Bernard Carr. I do not regard these remarkable relics merely as an ingenious theoretical possibility, but as an ever more compelling constituent of the actual Universe. Far from constituting an exotic afterthought, PBHs arise quite naturally in a great variety of inflationary and post-inflationary scenarios. Most remarkably of all, they may comprise an appreciable fraction—and, within certain allowed mass windows, perhaps even the entirety—of dark matter. For fuller accounts, I refer the reader to my seven PBH reviews and review chapters [19, 31, 38, 40, 41, 47, 50]. Nor ought PBHs to be regarded simply as rivals to particle dark matter: dark matter may be both microscopic and macroscopic, with a rich and fruitful interplay between its two manifestations. Nature may well have availed herself of both.

Most importantly—and most excitingly—the case for PBHs no longer rests upon a solitary anomaly. An extraordinary and ever-growing body of observations now points with increasing force in their direction. Each PBH interpretation set forth below admits of conventional astrophysical alternatives and remains subject to uncertainties of modelling; taken together, however, their pattern is astonishing. They extend over many orders of magnitude in mass, widely separated cosmic epochs and wholly different observational messengers, yet can all be accommodated within the selfsame natural PBH framework. It becomes ever harder to dismiss so remarkable a concordance as mere coincidence. In my judgement, these observations already amount to a compelling, and swiftly strengthening, case that primordial black holes exist. The principal positive indications and especially promising tests may be set forth as follows:

1. six exceedingly brief OGLE microlensing events, which can be fitted by Earth-mass PBHs contributing roughly one per cent of the dark matter density; it must nevertheless be conceded that free-floating planets remain a possible explanation, whilst a later high-cadence OGLE analysis towards the Magellanic Clouds placed strong constraints upon this PBH interpretation [E1], [E8];
2. quasar microlensing which has been construed as evidence for compact objects in galactic haloes, most especially in systems wherein the expected stellar microlensing optical depth is small; certain other analyses, however, remain consistent with ordinary stars [E2];
3. eight highly intriguing microlensing candidates in the putative 2–5 M⊙ lower mass gap, although they may yet prove to be ordinary stellar remnants [E3];
4. the significant large-scale cross-correlation between source-subtracted cosmic infrared and soft X-ray background fluctuations, for which accreting black holes at high redshift—including PBHs—furnish an especially attractive possible account [E4];
5. the apparent lower envelope, near a half-light radius of approximately 15 pc, in the observed population of ultra-faint dwarf galaxies, which may bear the imprint of dynamical heating by solar-mass compact objects; the unusually compact UMa III/UNIONS 1 may constitute an exception, although whether it be a dwarf galaxy or a star cluster has yet to be settled [E5], [E9];
6. the masses, spins and merger-rate distribution of binary black holes observed by LIGO–Virgo–KAGRA, now charted upon an unprecedented scale in GWTC-5.0, thereby furnishing ever greater power to distinguish between astrophysical and primordial channels of formation [E6];
7. the astonishingly swift appearance of massive—and, in some instances, extremely overmassive—black holes in the early Universe, which renders the seed problem yet more acute and has lent powerful impetus to scenarios in which primordial black holes themselves furnish the seeds [E10], [52];
8. the first direct dynamical weighing of a black hole beyond z = 5—and surely amongst the most arresting objects yet brought to light—Abell 2744–QSO1 at z = 7.04: a black hole of some fifty million solar masses, at least twice as massive as all the stars which observation permits its host to contain, whilst the gas about it possesses scarcely half of one per cent of the Sun’s metallicity. So extraordinary a conjunction sits most uneasily with the customary formation scenarios; yet, most remarkably, our dedicated simulations show that it may arise quite naturally, provided the system be seeded by a massive PBH [E10], [52];
9. the remarkable profusion of luminous galaxies at redshifts above ten, with spectroscopic confirmation now reaching z = 14.44, which affords a further and most promising means of testing PBH-assisted structure formation [48], [E7].

These observations and tests, together with numerous other possible positive indications for the existence of PBHs, are treated at length in my Physics Reports article [41]. Taken as a whole, they have borne the PBH hypothesis far beyond the province of mere speculation. Amongst the most decisive signatures would be those which ordinary stellar evolution cannot readily imitate—most notably a merger containing a securely identified subsolar-mass black hole—or the inference, from several independent probes, of one consistent PBH abundance. GWTC-5.0 already contains 390 cumulative transient candidates having an astrophysical probability of at least 50 per cent through O4b [E6]; the long-anticipated age of abundant gravitational-wave observations is therefore already upon us. My conviction could scarcely be plainer: I firmly believe that primordial black holes exist. I believe, moreover, that the swiftly growing convergence of gravitational-wave, microlensing and high-redshift observations will ere long furnish the first decisive and unambiguous evidence for them. The question, as I see it, is no longer whether PBHs merit serious consideration, but which observation will first establish their existence beyond reasonable doubt. A still grander question—whether they comprise an appreciable fraction, or perhaps even all, of dark matter—must await a determination of their cosmological abundance. Were the evidence ultimately to converge in this direction, we should thereby have discovered not only primordial black holes, but a macroscopic constituent of the dark Universe—surely one of the profoundest discoveries in modern cosmology.

The remarkable breadth of the subject is reflected in my own work, which may broadly be arrayed under five closely allied heads:

• Formation and Mass Functions [17–19, 25, 26, 31, 33, 35–38, 40, 46, 50]
• Astrophysical Signatures and Constraints [17, 19–24, 27, 29–35, 37, 38, 40, 41, 43–45, 47–56]
• Cosmic Structure and Black Hole Seeding [32, 33, 38, 40, 41, 47, 48, 50, 52]
• The Interplay between Microscopic and Macroscopic Dark Matter [23, 24, 32, 34, 38–40, 51, 53, 54]
• Quantum Structure and Evaporation [16, 39, 42, 49, 51, 53, 54]

Numbers without an “E” prefix denote my own contributions; see Publications.

Selected External References

• [E1] H. Niikura et al., Phys. Rev. D 99, 083503 (2019).
• [E2] M. R. S. Hawkins, Astron. Astrophys. 633, A107 (2020); E. Mediavilla et al., Astrophys. J. Lett. 836, L18 (2017).
• [E3] L. Wyrzykowski and I. Mandel, Astron. Astrophys. 636, A20 (2020).
• [E4] N. Cappelluti et al., Astrophys. J. Lett. 847, L11 (2017).
• [E5] S. Clesse and J. García-Bellido, Phys. Dark Universe 22, 137–146 (2018).
• [E6] LIGO Scientific Collaboration, Virgo Collaboration and KAGRA Collaboration, “GWTC-5.0: Observations from the Second Part of the Fourth LIGO–Virgo–KAGRA Observing Run and Updates to the Gravitational-Wave Transient Catalog” (2026); “GWTC-5.0: Population Properties of Merging Compact Binaries” (2026); see likewise the GWTC-5.0 catalogue introduction.
• [E7] R. P. Naidu et al., Open J. Astrophys. 9 (2026).
• [E8] P. Mróz et al., Astrophys. J. Lett. 976, L19 (2024).
• [E9] S. Devlin, H. Baumgardt and S. M. Sweet, Mon. Not. R. Astron. Soc. 539, 2485–2497 (2025).
• [E10] I. Juodzbalis et al., Nature 653, 1017–1021 (2026); R. Maiolino et al., Mon. Not. R. Astron. Soc. 548, staf2109 (2026).