In 2023, a subatomic particle smashed into the Mediterranean Sea with enough energy to rattle the foundations of physics. The particle was a neutrino, a fundamental subatomic particle that usually slips through the Earth unnoticed. Approximately 100 trillion neutrinos pass through your body every single second. These nearly massless, electrically neutral “ghost particles,” primarily originate from the sun and cosmic events.
But this one was different. It slammed into the detectors of the KM3NeT experiment with an energy of around 220 PeV — roughly 100,000 times more energetic than anything produced by the Large Hadron Collider.
For astrophysicists, this detection was a “smoking gun” that didn’t fit the crime scene. There are no known astrophysical sources capable of firing a neutrino with that specific energy profile. Even more perplexing, if such sources were common, the massive IceCube observatory in Antarctica, which has been watching the sky for over a decade, should have seen them. But IceCube saw nothing of the sort.
Now, a team of physicists from the University of Massachusetts Amherst has proposed a solution that is as elegant as it is exotic. They suggest that we didn’t see a standard cosmic ray or a dying star. Instead, we may have witnessed the violent death rattle of a “quasi-extremal primordial black hole” — a tiny, ancient, and electrically charged beast from the dawn of time that could explain the nature of dark matter.
The Fossils of the Big Bang
To understand why this specific neutrino is so disruptive, we have to look back to the first split-second of the universe. In 1966, Soviet physicists Yakov Zel’dovich and Igor Novikov proposed that the Big Bang was so chaotic that some pockets of space-time might have been dense enough to collapse directly into black holes.
Unlike the stellar black holes we see today, which form when massive stars die, these “primordial black holes” (PBHs) could be incredibly small — some the size of an atomic nucleus. Stephen Hawking later realized that these objects wouldn’t live forever. Due to quantum effects near their event horizons, they would slowly leak particles into space, a phenomenon now known as Hawking radiation.
This process leads to a runaway explosion. “The lighter a black hole is, the hotter it should be and the more particles it will emit,” explains Andrea Thamm, a physicist at UMass Amherst and co-author of the new study. “As PBHs evaporate, they become ever lighter, and so hotter, emitting even more radiation in a runaway process until explosion”.
If these explosions are happening today, they should be spraying the cosmos with high-energy particles. This is where the trouble begins.
The Tale of Two Detectors
When KM3NeT spotted its monster neutrino (event KM3-230213A), astrophysicists were scrambling for an explanation. If standard primordial black holes are exploding often enough for KM3NeT to catch one, IceCube should have caught dozens.
IceCube is a larger, more established detector. If the universe were filled with standard exploding black holes, the sky should be lit up with high-energy neutrinos. But IceCube’s data suggested otherwise. This discrepancy created a “3.5 sigma tension” between the experiments — scientific shorthand for “something is seriously wrong with our model”.
The UMass Amherst team realized that the problem wasn’t the detectors; it was our assumption about the black holes. They proposed that these aren’t your garden-variety Schwarzschild black holes. They are “charged” black holes hiding in a dark sector.
The Dark Sector Valve
In their new paper, the researchers introduce a fascinating explanation for the 2023 event: what if these primordial black holes carry a “dark charge”?
We know that 85% of the matter in the universe is “dark matter,” invisible stuff that doesn’t interact with light. The researchers posit that just as normal matter has electromagnetism, dark matter might have its own “dark electromagnetism” governed by a “dark u(1) symmetry.” The latter is a theoretical extension of the Standard Model (SM) that introduces a new abelian gauge symmetry acting on a hidden “dark sector”.
If a primordial black hole formed with a small amount of this dark charge, its life cycle would change dramatically.
Cosmic Coma
As the primordial black hole evaporated and shrank, the density of its dark charge would skyrocket. Eventually, the black hole would become “quasi-extremal” — a state where the electrical repulsion balances out the gravitational crush.
In this state, it essentially stops evaporating. The black hole enters a coma, becoming “cosmologically long-lived”. It sits there, tiny and heavy, until the dark electric field at its surface becomes so intense that it rips space-time apart, creating pairs of dark electrons. This is called the “dark Schwinger effect.”
Once this effect kicks in, the black hole discharges rapidly and explodes. Crucially, the UMass team calculated that this specific type of explosion suppresses the emission of neutrinos at the 1 PeV energy range (where IceCube is most sensitive) but allows them to blast out at 100 PeV (where KM3NeT saw its event).
By tweaking the “dark charge” and the mass of the “dark electron,” the researchers found a sweet spot where the data from both detectors align perfectly. The massive discrepancy vanishes.
A Candidate for Dark Matter?
If these quasi-extremal black holes exist, they solve a much bigger puzzle.
Previously, astronomers have hunted for the source of dark matter. But the new study finds that a population of these objects could “constitute the entirety of dark matter in the universe”.
Standard primordial black holes have largely been ruled out as dark matter candidates because their explosions would produce too much background gamma radiation — a glow that telescopes like HAWC would have seen. But because these charged black holes spend most of their lives in a dormant, quasi-extremal state, they don’t emit that background glow. They hide in the shadows until their final, violent moments.
This aligns with what other theorists are beginning to suspect. “They’re one of the few good theories for what dark matter could be,” notes Wenzer Qin, a theoretical physicist at NYU, regarding primordial black holes.
The Next Step
If the UMass Amherst team is right, we are sitting in a universe filled with tiny, charged black holes that occasionally pop like cosmic firecrackers. The next decade will be the proving ground. The researchers predict that with the unique signature of these “dark” explosions identified, we might soon spot more of them.
“If such an explosion were to be observed,” Thamm notes, “it would give us a definitive catalog of all the subatomic particles in existence,” revealing physics that has remained hidden since the Big Bang.
The findings appeared in the journal Physical Review Letters.