Even enormous primordial black holes cannot be dark matter’s hidden source

Black holes heavier than 100 billion Suns, a class the physicists call stupendously large, cannot make up a significant share of dark matter, according to a team of theoretical physicists whose analysis of the cosmic microwave background closes off the heaviest mass range proposed for primordial black holes. The result was reported by Nature on October 29, 2026. The largest black hole known weighs about 36 billion solar masses, and nothing in the stupendously large class has ever been detected.

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The finding removes one candidate for dark matter and leaves the question of what dark matter is where it was.

Why a few giant black holes would show up in the sky

Primordial black holes are objects that could have formed in the first instants after the Big Bang, before stars existed. They are a long-standing candidate for dark matter because they would be dark, would pull on ordinary matter, and would not need any new particle. The paper, titled Closing the Mass Window for Stupendously Large Black Holes, is by Christopher Gerlach, Yann Gouttenoire, Antonio Iovino and Nicholas Leister. It targets the extreme end, black holes of roughly 10 to the 11th solar masses or more.

The reasoning rests on how matter was spread out early. Dark matter that is made of a few gigantic objects is lumpy in a way that dark matter made of countless small particles is not. Gerlach told Space.com that in such a case the universe becomes grainy on very large scales. The team says the arXiv version shows such black holes would produce isocurvature perturbations, patchy variations in the density of matter that do not track the radiation, larger than the limits set by the Planck satellite’s measurements of the cosmic microwave background.

One part in 100,000 across the cosmic microwave background

The cosmic microwave background is the oldest light in the universe, and its temperature varies across the sky by about one part in 100,000. Iovino described that variation in the Space.com report, and it is the number against which the graininess is judged. The Nature report says the same thing in its own terms: black holes of this size would have left a bigger imprint on the radiation than observed.

Leister said the black holes therefore cannot be a significant part of the dark matter. The mass range at stake is hard to picture. The team examined objects above 100 billion solar masses, and the heaviest of them, in the range of a million billion to 10 million billion solar masses, would outweigh entire galaxies.

The limits the paper tests against come from the Planck satellite’s measurements of the microwave background, described in the 2018 Planck results paper. The new analysis compares a theoretical prediction to those existing measurements. Planck’s results put dark matter at roughly 26 percent of the universe’s contents and ordinary matter at about 5 percent, which is why a candidate that fails the microwave background test matters: the missing material is the larger share of all matter, and any explanation has to account for it without leaving a fingerprint on the oldest light.

A narrowed window, not a closed case

The authors take care over what they are not claiming. They stress that the result does not say such black holes cannot exist, only that they are not a major fraction of dark matter. A handful of very heavy objects might lurk in the cosmos without mattering for the missing mass, which astronomers infer from the motions of galaxies and the bending of light around galaxy clusters.

The authors are theorists at the Mainz Institute for Theoretical Physics in Germany, apart from Iovino, who is at New York University Abu Dhabi. Their approach is a consistency check, not a telescope search: no instrument has to find or miss a black hole of this size, because the microwave background already contains the answer, and it shows no such graininess. A claim that dark matter hides in a few monsters would have to explain why the oldest light in the sky is so smooth.

The practical effect is on the theorists’ menu. Before the paper, the heaviest primordial black holes could be listed among the possible dark matter candidates on the strength of their having never been ruled out. After it, anyone who proposes them has to explain how they evade the Planck limit, and no such explanation appears in the sources reviewed here.

The study is scheduled to appear in Physical Review Letters, where Space.com places the paper’s publication on October 26, with its article dated October 23.

The team is now checking whether primordial black holes at the opposite extreme, asteroid-sized ones, can be ruled out as a major component of dark matter. That mass window is still open in the literature, and it is where the argument over primordial black holes will play out. If asteroid-mass black holes also fail the test, the explanation will have to come from a particle that no laboratory has yet found.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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