How to look for black holes that formed before the first stars

A black hole that needs no star

The black holes we know best are what is left after a massive star dies, and they weigh at least a few times the Sun. The early universe offers another route. In its first second it was filled with hot radiation that was almost perfectly smooth. Suppose one region was denser than its surroundings by about forty or fifty percent. While the region was larger than the distance light had travelled since the beginning, nothing could pull it together. Once that distance grew to enclose it, gravity could act on the whole region at once, and if the excess was large enough the region collapsed straight into a black hole.

The mass of such a black hole is set by the time it formed. It is roughly the mass inside that light-travel distance, about two hundred thousand solar masses for every second after the beginning. A black hole made $10^{-15}$ seconds in would weigh about as much as a large asteroid and be about the size of a virus. One made a thousandth of a second in would weigh about forty Suns. These are called primordial black holes.

Reading: Particle Data Group · Review 27, Dark Matter (2025 edition, 1 December 2025) · Harada, Yoo & Kohri · Threshold of primordial black hole formation · arXiv:1309.4201v4, Phys. Rev. D 88, 084051 (2013)

Why they could be the dark matter

Galaxies move as if they contain much more mass than we can see. Whatever supplies it barely emits or absorbs light and interacts mainly through gravity. A population of black holes formed before any star would behave that way, so primordial black holes are a long-standing dark-matter candidate. Physicists describe their abundance by the fraction of the dark matter they would make up, from zero to one.

Reading: Particle Data Group · Review 27, Dark Matter (2025 edition, 1 December 2025) · Carr, Iovino, Perna, Vaskonen & Veermäe · Primordial black holes: constraints, potential evidence and prospects · arXiv:2601.06024v2 (1 April 2026)

Four signals that would give them away

The lightest ones would announce themselves by evaporating. Stephen Hawking showed that a black hole slowly radiates, and the smaller it is the hotter it glows. One born with about $5\times10^{14}$ grams, the mass of a small mountain, would be finishing its evaporation about now. Somewhat heavier ones would still be emitting gamma rays. The measured gamma-ray background and the amounts of light elements made in the first minutes leave little room for black holes up to about $10^{17}$ grams.

Heavier ones would bend light. When a compact mass passes almost in front of a star, it acts as a lens and the star brightens for a while, a microlensing event. A survey camera on the Subaru telescope watched stars in the Andromeda galaxy for brief flickers of this kind. The OGLE survey watched nearly 80 million stars in the Large Magellanic Cloud for twenty years and found no events lasting more than a year. That rules out black holes between about $2\times10^{-4}$ and 6 solar masses as more than one percent of the dark matter.

The heaviest ones would heat gas. A black hole of more than about ten solar masses pulls in gas, and the gas glows as it falls. In the young universe that glow would have changed the moment when hydrogen became neutral, which shows up in the cosmic microwave background, the oldest light we can see. How strongly this rules them out depends on how the gas actually falls in, which is still debated.

Pairs would merge. Primordial black holes that happen to form close together can pair up early and merge billions of years later, sending out gravitational waves. The LIGO, Virgo and KAGRA detectors have recorded many black-hole mergers. An analysis of their third observing run limits black holes between 1 and 200 solar masses to at most about a thousandth of the dark matter.

Reading: Carr, Iovino, Perna, Vaskonen & Veermäe · Primordial black holes: constraints, potential evidence and prospects · arXiv:2601.06024v2 (1 April 2026) · Carr, Kohri, Sendouda & Yokoyama · New cosmological constraints on primordial black holes · arXiv:0912.5297v2, Phys. Rev. D 81, 104019 (2010) · Smyth, Profumo, English, Jeltema et al. · Updated constraints on asteroid-mass primordial black holes as dark matter · arXiv:1910.01285v2, Phys. Rev. D 101, 063005 (2020) · Mróz, Udalski et al. · No massive black holes in the Milky Way halo · arXiv:2403.02386v3, Nature 632, 749 (2024) · Serpico, Poulin, Inman & Kohri · Cosmic microwave background bounds on primordial black holes including dark matter halo accretion · arXiv:2002.10771v2, Phys. Rev. Research 2, 023204 (2020) · Andrés-Carcasona, Iovino, Vaskonen & Veermäe · Constraints on primordial black holes from LIGO-Virgo-KAGRA O3 events · arXiv:2405.05732v2, Phys. Rev. D 110, 023040 (2024)

The mass range that is still open

Between the evaporation limits and the microlensing limits lies a gap from about $10^{-17}$ to $10^{-10}$ solar masses, the masses of asteroids. These black holes are too heavy to have evaporated and too light to lens a star noticeably: they are much smaller than the stars behind them, and at that size light behaves as a wave and spreads around the lens. Proposed limits from neutron stars and white dwarfs that swallow or are ignited by black holes have been questioned in later work. In this range primordial black holes could still be all of the dark matter.

Two ideas could close or confirm the window. Very compact X-ray stars are small enough to show microlensing by asteroid-mass lenses. And if such black holes are everywhere, a few per decade should fly through the Solar System and nudge the planets by an amount that precise tracking of Mars or Venus might detect.

Reading: Carr, Iovino, Perna, Vaskonen & Veermäe · Primordial black holes: constraints, potential evidence and prospects · arXiv:2601.06024v2 (1 April 2026) · Particle Data Group · Review 27, Dark Matter (2025 edition, 1 December 2025)

A spread of masses must be checked as a whole

Each published limit assumes every black hole has the same mass. A real formation process makes a spread. To test that spread, each observation has to add up the contribution of every mass with its own sensitivity, one observation at a time. Laying the spread over the drawn limit curves and checking that it stays below all of them gives the wrong answer, because a limit curve caps the total abundance while the spread is an amount per range of mass.

The early density peak that forms the black holes matters too. Only an extremely rare high tail of the density distribution collapses. In a simple model, lowering the peak by fifteen percent cuts the number of black holes by about a hundred times.

Reading: Carr, Raidal, Tenkanen, Vaskonen & Veermäe · Primordial black hole constraints for extended mass functions · arXiv:1705.05567v3, Phys. Rev. D 96, 023514 (2017) · Carr, Iovino, Perna, Vaskonen & Veermäe · Primordial black holes: constraints, potential evidence and prospects · arXiv:2601.06024v2 (1 April 2026) · Young, Byrnes & Sasaki · Calculating the mass fraction of primordial black holes · arXiv:1405.7023v4, JCAP 1407 (2014) 045

What has been seen so far

No primordial black hole has been identified. Some observations keep the idea alive. A few gravitational-wave events have masses that dying stars find hard to produce. One candidate signal suggested a merger involving an object lighter than the Sun. Patterns in the faint infrared and X-ray glow left after removing known sources have been read as a sign of black holes in early galaxies. Each of these has astrophysical explanations as well.

So primordial black holes remain a candidate. The limits show where they cannot be hiding, and the asteroid-mass window shows where they still could be.

Reading: Carr, Iovino, Perna, Vaskonen & Veermäe · Primordial black holes: constraints, potential evidence and prospects · arXiv:2601.06024v2 (1 April 2026) · Andrés-Carcasona, Iovino, Vaskonen & Veermäe · Constraints on primordial black holes from LIGO-Virgo-KAGRA O3 events · arXiv:2405.05732v2, Phys. Rev. D 110, 023040 (2024)