On November 12, 2025, the LIGO-Virgo-KAGRA gravitational wave observatories detected a signal unlike any they had seen before. Designated S251112cm, the merger had a chirp mass between 0.1 and 0.87 solar masses, far too light to be a pair of ordinary stellar-mass black holes, and with only an 8 percent probability of involving a neutron star.
In a study published in The Astrophysical Journal, astrophysicists Nico Cappelluti and Alberto Magaraggia of the University of Miami argue that the signal may be the first direct evidence of primordial black holes, objects formed not from collapsing stars but from density fluctuations in the first fractions of a second after the Big Bang. And if they are right, these ancient black holes could account for a significant portion, if not all, of the universe’s dark matter.
“This is very strong evidence that these types of black holes exist,” Cappelluti said. “But we’ll need to detect another such signal or even several others to get the smoking-gun confirmation.”
What makes it unusual
Stellar-mass black holes form from the collapse of massive stars in supernovae, and the lightest such black holes known weigh roughly 3 to 5 solar masses, the lower boundary set by the physics of neutron star collapse. A merger with a chirp mass below one solar mass has no conventional astrophysical explanation. Neutron star mergers are possible, but the Bayesian analysis of S251112cm gives only an 8 percent probability of a neutron star being involved. The objects are almost certainly black holes below the stellar-collapse mass threshold.
The signal was detected across all three LVK detectors (LIGO Hanford, LIGO Livingston, and Virgo) with a log-Bayes coherence factor of +6.1, strong evidence of a genuine gravitational wave event. Its false alarm rate is roughly one in 6.2 years. The estimated distance is 93 plus or minus 27 megaparsecs (approximately 303 million light-years). No credible kilonova counterpart was found in electromagnetic follow-up searches, consistent with a black hole-black hole merger.
The primordial black hole case
Primordial black holes are hypothesized to have formed during the QCD epoch, a phase transition in the first microseconds after the Big Bang when quarks combined to form protons and neutrons. The physics of this transition briefly softened the equation of state of the early universe, allowing regions of extreme density to collapse directly into black holes across a vast mass range.
The model used by Cappelluti and Magaraggia, based on an extended mass spectrum from Hasinger (2020) modified by lepton-flavor asymmetries, predicts four characteristic mass populations: planetary-mass black holes from the electroweak transition; Chandrasekhar-scale black holes around 1.5 solar masses from baryon appearance; approximately 50-solar-mass black holes from pion formation; and supermassive black holes from electron-positron annihilation.
S251112cm, with its chirp mass in the 0.1-0.87 solar mass range, fits cleanly into the low-mass tail of the Chandrasekhar-scale primordial population.
The model predicts a detectable subsolar merger rate of 0.8 per year at LIGO’s O3b sensitivity. The single detection of S251112cm gives an observed rate of 0.23 per year (with a 95 percent confidence interval of 0.012 to 1.09 per year), statistically consistent with the prediction. The same model also predicts approximately 120 mergers per year in the 3 to 200 solar mass range, matching LIGO’s observed rate, which suggests that some fraction of LIGO’s known black hole mergers may also be primordial.
What it means for dark matter
The model’s predicted primordial black hole fraction in the observable mass range (10^{-6} to 4 x 10^{8} solar masses) is f_PBH = 0.339, meaning roughly 34 percent of dark matter in this mass window could be made of these objects. Extending the mass function down to sub-planetary scales, the model can account for 100 percent of dark matter.
“Our research indicates that these primordial black holes could account for a significant portion, if not all, of dark matter,” Cappelluti said.
The detection also provides a lower limit: from this single event, at least 4 percent of dark matter in the relevant mass range must be in the form of primordial black holes, at 95 percent confidence.
Alternatives and caution
Not everyone is convinced. The signal could still be a statistical fluctuation, though the high coherence factor makes that unlikely. An alternative explanation, the “superkilonova” model, where subsolar-mass neutron stars form from fragmentation in collapsar accretion disks during supernovae, was tested in a follow-up study that found a coincident Type IIb supernova two days before S251112cm. But the chance coincidence probability is 2 to 9 percent, and the evidence was deemed “suggestive but inconclusive.”
“We predict that subsolar black holes like the one LIGO may have observed should indeed be rare, consistent with how infrequently such events have been seen so far,” said Alberto Magaraggia. “We caution that this event may still be revised or retracted.”
Another detection of a similar subsolar-mass merger would be needed to confirm the primordial black hole interpretation. LIGO’s current observing run, O4, continues to collect data, and the upgraded detectors may be sensitive enough to catch more of these elusive signals.