An unusual gravitational wave signal detected late last year by the Laser Interferometer Gravitational-Wave Observatory (LIGO) has renewed hopes that primordial black holes — long considered purely theoretical — may finally be within reach of discovery, potentially solving one of astronomy's greatest mysteries: the nature of dark matter.
Primordial black holes are thought to have formed during the first fraction of a second after the Big Bang, long before the first stars or galaxies existed. Unlike the black holes created by collapsing stars, which typically range from several times the mass of the Sun to billions of solar masses, these hypothetical objects could be as small as an asteroid.
An unexplained signal
In November 2025, LIGO issued an automated alert for a merger in which at least one object appeared to have less than one solar mass — far too small to be explained by conventional stellar evolution. Such a low-mass black hole has no standard astrophysical explanation and could instead point to a primordial origin.
"We believe our study will aid in confirming that they actually do exist," said Nico Cappelluti, an associate professor in the University of Miami's Department of Physics, who conducted the research with Ph.D. student Alberto Magaraggia.
Not everyone in the scientific community is convinced. Some astrophysicists have suggested the signal could simply be noise within LIGO's extremely sensitive detectors rather than evidence of a remarkable new discovery.
A solution to dark matter
Cappelluti and Magaraggia argue that the detected object is best explained as a primordial black hole. To test this, they estimated how many primordial black holes might exist throughout the cosmos and how frequently LIGO should detect them. Their findings, published in The Astrophysical Journal, suggest that the mysterious LIGO signal has no conventional astrophysical explanation and is most consistent with a primordial black hole.
"Our research indicates that these primordial black holes could account for a significant portion, if not all, of dark matter," Cappelluti said.
Dark matter is the invisible substance that makes up approximately 85% of all matter in the universe, providing the gravitational pull that helps hold galaxies together. Despite decades of searching, its nature remains unknown.
A theory dating back to Hawking
The concept of primordial black holes dates back to the Cold War era, when Soviet scientists Yakov Zeldovich and Igor Novikov first proposed their existence. In the early 1970s, Stephen Hawking expanded on the idea, suggesting these objects could be abundant throughout the universe, emit radiation (now known as Hawking radiation), and possibly explain dark matter.
LIGO's 2015 detection of gravitational waves — ripples in spacetime predicted by Einstein's general theory of relativity — opened the first observational window to test these theories.
What comes next
Both researchers emphasize that one detection alone is not enough. "LIGO picked up what is very strong evidence that these types of black holes exist. But we'll need to detect another such signal or even several others to get the smoking-gun confirmation," Cappelluti said.
Planned upgrades will make LIGO even more sensitive, increasing its chances of finding additional candidate primordial black holes. Future observatories will extend that reach much further. The European Space Agency's Laser Interferometer Space Antenna (LISA), scheduled for launch in 2035, is expected to detect gravitational waves from the universe's earliest epochs. Another planned facility, Cosmic Explorer, currently in the design phase in the United States, is expected to be about 10 times more sensitive than LIGO.
For now, scientists wait — and watch the data — hoping for the next signal that could confirm one of the most profound discoveries in modern astronomy.
