An extraordinary neutrino that struck a detector beneath the Mediterranean Sea in 2023 has left scientists searching for an equally extraordinary explanation. Now, researchers have proposed a possibility that sounds like science fiction: the particle may have been produced by an ancient black hole associated with a hypothetical fifth dimension.

The event, known as KM3-230213A, was recorded on February 13, 2023, by the KM3NeT/ARCA neutrino telescope. Scientists estimate that the neutrino carried an energy of roughly 220 petaelectronvolts (PeV), making it the most energetic neutrino ever detected.

That is roughly 30 times the energy of the previous record-setting neutrino and around 30,000 times the energy reached by protons in the Large Hadron Collider. But the particle’s extraordinary energy is only half the mystery.

Astronomers have not been able to identify a convincing object in the direction from which it came.

A new theoretical study suggests that the missing source could involve primordial black holes formed from cosmic strings in an extra dimension. The idea is highly speculative, but it offers a possible explanation for another unusual feature of the event: the absence of an accompanying electromagnetic signal.

What was the 2023 KM3NeT neutrino?

KM3-230213A was not detected as a neutrino directly.

Instead, the neutrino interacted near the detector and produced a high-energy muon, a heavier relative of the electron. That muon traveled through the seawater surrounding KM3NeT/ARCA and generated flashes of Cherenkov light that the detector’s optical sensors could record.

The event was extraordinary.

During a window of only about two microseconds, the detector registered 28,086 hits across its optical sensors. Some photomultiplier tubes were overwhelmed by the amount of light. Scientists reconstructed the muon’s energy at approximately 120 PeV, which implies that the original neutrino had an energy of around 220 PeV.

That makes KM3-230213A the highest-energy neutrino ever observed.

Why are neutrinos so difficult to trace?

Neutrinos are often called “ghost particles” because they interact extraordinarily weakly with matter.

That is also what makes them scientifically valuable.

Unlike light, neutrinos can travel enormous cosmic distances without being absorbed or deflected significantly by matter and magnetic fields. In principle, they can therefore carry information directly from violent environments such as active galaxies and exploding stars.

The problem is that the same ability makes them extremely difficult to detect.

KM3NeT did not see the neutrino traveling through space. It inferred its existence from the particle it created near the detector.

Where did KM3-230213A come from?

That remains unknown.

The KM3NeT Collaboration searched the region of sky corresponding to the particle’s arrival direction but found no compelling Galactic or extragalactic source. Possible candidates include active galactic nuclei, gamma-ray bursts and other powerful cosmic accelerators. Cosmogenic neutrinos, produced when ultra-high-energy cosmic rays interact with background radiation, are another possibility.

Blazars have also been investigated as possible sources.

But no explanation has yet emerged as a confirmed answer.

That uncertainty is particularly interesting because high-energy particle production generally comes with other observable signatures.

If an astrophysical object produces an extremely energetic neutrino, related processes can also generate electromagnetic radiation, including gamma rays. Yet astronomers did not identify an obvious electromagnetic counterpart in the direction of KM3-230213A.

That missing signal has become one of the central clues in the search for its origin.

What is the new fifth-dimension black hole theory?

A team including physicist Dieter Lüst has proposed an explanation involving primordial black holes and an extra dimension.

The study, Neutrinos from Primordial Black Holes in Theories with Extra Dimensions, examines whether the event could have been produced by the evaporation of a five-dimensional primordial black hole. The paper was posted as a preprint in 2025.

The idea builds on a theoretical framework known as the dark dimension scenario.

In ordinary experience, the universe has three dimensions of space plus time. String theory, however, allows for additional spatial dimensions.

The proposed “dark dimension” would be a hypothetical fifth dimension that is inaccessible to ordinary matter but can influence physics through gravity.

It has not been observed.

That point is crucial: the fifth dimension in this theory is a mathematical possibility, not a newly discovered part of the universe.

How could cosmic strings create a black hole?

The theory becomes even more unusual when cosmic strings enter the picture.

Cosmic strings are hypothetical one-dimensional defects that could have formed during phase transitions in the early universe. They are not ordinary strings in space; rather, they would represent extremely thin concentrations of energy associated with the structure of spacetime.

Under certain theoretical conditions, cosmic strings could form loops.

Those loops could eventually collapse and produce primordial black holes.

This differs from the more familiar picture in which primordial black holes form from unusually dense regions of the early universe shortly after the Big Bang.

In the extra-dimensional scenario, the black holes would exist in a five-dimensional framework.

Why would these black holes produce neutrinos?

Black holes are not completely black under quantum physics.

Stephen Hawking’s theoretical work showed that black holes can emit radiation and gradually lose mass in a process now known as Hawking evaporation.

As a black hole becomes smaller, its temperature rises. Near the final stages of evaporation, it could potentially emit extremely energetic particles.

The researchers argue that five-dimensional primordial black holes could have properties that make them particularly interesting for the KM3-230213A event.

According to the study, such black holes would be larger, colder and longer-lived than four-dimensional primordial black holes of the same mass. Near the end of their lives, however, they could produce a burst of high-energy Standard Model particles, including neutrinos.

Why is the absence of gamma rays important?

This may be the most intriguing part of the proposal.

Ordinary astrophysical processes that produce high-energy neutrinos can also produce photons. Those photons could then be detected by telescopes looking at the same region of sky.

But KM3-230213A arrived without an obvious electromagnetic counterpart.

The five-dimensional primordial black hole scenario offers a potential explanation.

The researchers argue that these hypothetical black holes could decay predominantly through gravitationally coupled modes that are effectively invisible to observers confined to the familiar four-dimensional universe. A final burst could nevertheless produce Standard Model particles that reach detectors such as KM3NeT.

In other words, the model could potentially explain both clues at once:

• Why was the neutrino so energetic?
• Why was there no obvious accompanying electromagnetic source?

That does not prove the theory is correct. It simply makes the hypothesis worth investigating.

Are primordial black holes the only black-hole explanation?

No.

Another 2025 Physical Review D study investigated whether KM3-230213A could have originated from an evaporating primordial black hole without necessarily invoking the same extra-dimensional scenario. That research found that a black hole capable of producing particles in the approximate energy range of the event would need to be extremely small, with a mass below about $10^7$ grams under the model considered.

Other researchers have explored conventional astrophysical explanations.

One 2026 Physical Review D paper examined whether the event could have come from a long-duration astrophysical transient rather than representing part of a diffuse neutrino background. The researchers found that a transient source lasting up to roughly two years could be consistent with the available observations under certain assumptions.

So the scientific picture currently looks less like a single answer and more like a list of competing possibilities.

Why are scientists still puzzled by one neutrino?

The biggest problem is statistics.

KM3NeT has observed an extraordinary event, but one event does not establish a new population of cosmic objects.

A separate analysis published in Physical Review X found that the KM3NeT observation is in mild-to-moderate tension with the absence of similar ultra-high-energy events in IceCube and the Pierre Auger Observatory. Depending on the dataset and assumptions, the tension ranges from about 1.6 to 2.9 sigma — interesting, but not enough to claim a discovery.

More observations are needed.

If KM3NeT or other observatories detect additional neutrinos with comparable energies, scientists can begin asking whether these events follow a recognizable pattern.

If the events repeatedly arrive without electromagnetic counterparts, theories involving hidden or weakly interacting sources could become more compelling.

If they repeatedly point toward active galaxies or transient sources, conventional astrophysical explanations could win out.

What would prove the fifth-dimension theory?

Finding another ultra-high-energy neutrino would help, but it would not be enough on its own.

Scientists would need multiple independent observations that match the predictions of the model while competing explanations fail.

That could include:

• additional ultra-high-energy neutrinos;
• a characteristic energy distribution;
• evidence consistent with Hawking evaporation;
• a lack of corresponding electromagnetic radiation;
• a population of primordial black holes with the predicted properties; and
• independent evidence supporting the existence of the proposed extra dimension.

Until then, the fifth dimension remains exactly that — a theoretical possibility.

The mystery is bigger than one particle

KM3-230213A is important because it has pushed neutrino astronomy into an energy range scientists had barely explored.

The event is already forcing researchers to reconsider what kinds of cosmic environments can accelerate particles to extraordinary energies. It may eventually reveal a new class of astrophysical accelerator, provide evidence for primordial black holes, or simply turn out to be an exceptionally rare event from a known process.

The most dramatic possibility is that it could provide indirect evidence for physics beyond the familiar four-dimensional universe.

But scientists are not there yet.

For now, the most accurate description is also the most intriguing: an extraordinarily energetic neutrino arrived from an unidentified cosmic source, and one theoretical explanation says the culprit could be an ancient black hole associated with a hidden fifth dimension.

The next neutrino may be the one that tells us whether that idea belongs in a physics textbook — or back in the speculative pile.