Physicists have suggested many candidates for dark matter, and primordial black holes are a long-discussed one. The idea is that black holes formed early in the universe's history and could account for some or all of the missing mass. A paper published Oct. 26 in Physical Review Letters now addresses the top end of the mass range: the so-called "stupendously large" black holes.
The paper is titled "Closing the Mass Window for Stupendously Large Black Holes." Its authors are Gerlach, Gouttenoire, Iovino and Leister, who work at institutions in Mainz, Abu Dhabi (NYU Abu Dhabi) and Geneva. They conclude that primordial black holes heavier than roughly 1011 solar masses cannot make up a significant share of dark matter. Their evidence comes from the cosmic microwave background (CMB), the oldest light in the universe.
How big is "stupendously large"?
The name is not an exaggeration. Space.com's coverage of the study defines stupendously large black holes (SLBHs) as objects heavier than 100 billion solar masses, which is the same as 1011 in the paper's notation. For comparison, Space.com reports that the largest black hole known today may weigh about 36 billion suns. Any SLBH that contributed to dark matter would therefore be nearly three times heavier than the current record holder (100 billion divided by 36 billion is about 2.8).
Astronomers have not detected any black holes this large, according to Space.com. This team instead looked for an effect that such objects would have on the universe as a whole.
The argument from smoothness
The study's main point is about how uneven the early universe could have been. According to the paper's abstract, primordial black holes with masses above about 1011 solar masses would create isocurvature perturbations. These are a kind of irregularity in the early universe, and the ones produced by SLBHs would be larger than the limits that Planck observations of the CMB allow.
Space.com explains this in simpler terms. The CMB is extremely uniform, with temperature varying by only about one part in 100,000 across the sky. If a large share of dark matter were packed into a relatively small number of huge black holes, that mass would be clumped rather than smooth. Space.com quotes co-author Christopher Gerlach as saying such a universe "becomes grainy on very large scales." The CMB analysis found that it is not grainy the way these black holes would have made it. The authors conclude that the black holes, at least in amounts large enough to count as dark matter, are not there.
The argument rests on a well-measured observation rather than on any detection. The CMB has been mapped very precisely, and the team's result is that this high-mass population would have made it look measurably different from what Planck recorded.
A short paper with a long path to print
The result was posted first as a preprint on arXiv (arXiv:2508.08238) on Aug. 11, 2025. It was filed under cosmology and nongalactic astrophysics (astro-ph.CO) and is four pages long with two figures. The preprint carries the report number MITP-25-053. Physical Review Letters received the manuscript on Aug. 19, 2025, accepted it on Sept. 9, 2026, and published it on Oct. 26, 2026, as Phys. Rev. Lett. 137, 181401. The journal's editors selected the paper as an Editors' Suggestion, a label shown on the journal's page for the paper.
About 12 and a half months passed between submission and acceptance, according to the journal's dates, which also list a revision on March 30, 2026. The published record does not say what happened during review.
Why It Matters
Ruling candidates out is a large part of the search for dark matter. Every mass range that is excluded leaves less room for any particular explanation. That applies to particle candidates and to compact objects like black holes. This paper places a limit on the heaviest primordial black holes, based on Planck's CMB measurements.
The limit is specific. The paper does not claim that black holes this massive cannot exist. It says they cannot be a significant component of dark matter. Space.com reports the researchers stressed that the finding does not mean such black holes cannot exist, only that they do not make up a major fraction of dark matter.
The result also shows that CMB measurements can constrain hypothetical objects nearly three times heavier than any black hole astronomers have found.
What comes next
According to Space.com, the team is now investigating whether they can rule out the opposite end of the range: asteroid-size black holes. These would be very small primordial black holes.
For now, the takeaway is clear. If dark matter is made of primordial black holes, the CMB shows that they are not black holes of more than about 100 billion solar masses.
