'Stupendously Large' Primordial Black Holes Ruled Out as Dark Matter by Mainz Physicists
A Physical Review Letters analysis finds black holes of at least 100 billion solar masses would leave patterns in the cosmic microwave background that astronomers do not see.
One of the more exotic candidates for dark matter has just lost ground. A team of theorists has shown that primordial black holes weighing at least 100 billion times the mass of the Sun cannot make up the dark matter that holds galaxies together.
The paper, "Closing the Mass Window for Stupendously Large Black Holes," appears in Physical Review Letters (DOI 10.1103/qlf9-thf4) and was posted earlier as arXiv:2508.08238. The team is associated with the Mainz Institute for Theoretical Physics, with Nicholas Leister named as leading the work; the publication details list Christopher Gerlach as first author. The black holes in question would be larger than the supermassive black holes at the centers of galaxies, including the one in the Milky Way.
Primordial black holes are hypothetical objects that formed in the first moments after the Big Bang, rather than from collapsing stars. Dark matter makes up about 85% of the matter in the universe, and because primordial black holes would not be made of ordinary atoms, they have long been floated as a possible explanation.
The team's argument rests on statistics. If such black holes were scattered randomly through space, some regions would hold a surplus of them and others a shortage. As the universe expanded and radiation thinned out, those imbalances would turn into patchy variations in the ratio of dark matter to radiation. The researchers calculated how strong those variations would be and compared them with measurements of the cosmic microwave background, the oldest light in the universe.
A substantial population of these giants, they found, would make the dark-matter-to-radiation balance too uneven. The microwave background does not show the patterns that would result. For the calculation the team assumed that all black holes have the same mass.
The result also puts pressure on a second idea, that giant primordial black holes seeded the surprisingly massive early galaxies seen by the James Webb Space Telescope.
The authors are careful about the limits of the finding. It does not rule these objects out entirely, since populations with a wide spread of masses still need to be studied. The team expects the upcoming LiteBIRD and Simons Observatory microwave-background experiments to tighten the limits, and plans to push the analysis to even heavier black holes by looking for the gravitational waves they would have produced when they formed. The study closes one window, but dark matter's identity remains unsolved.
Primordial black holes are attractive to theorists partly because they need no new particles. Lighter ones, in the asteroid-mass range, are still debated as dark matter candidates, and other mass windows have been closed by gravitational lensing surveys and other observations over the years. The new paper deals with the opposite extreme, the heaviest objects anyone has seriously proposed, where the microwave background is the cleanest test. The result is reported by Phys.org writer Sam Jarman and is based on a published, peer-reviewed analysis.
