In February 2023, an underwater telescope called KM3NeT, anchored several miles beneath the Mediterranean Sea, recorded the brightest particle track ever seen in the universe. A single flash raced through the instrument’s glass spheres, and computer checks showed that the parent particle must have carried about 220 peta-electronvolts of energy. That figure is so large it dwarfs the beams at the Large Hadron Collider, the world’s most powerful accelerator, by almost one hundredfold.
Scientists first thought the visitor was an ultra-energetic neutrino. Neutrinos belong to an odd family of particles that glide through planets, people and even lead blocks with almost no resistance. Although they are hard to catch, detectors such as KM3NeT and its cousin the IceCube Neutrino Observatory, which sits frozen in the Antarctic ice, can see the faint blue glow that appears when a neutrino finally bumps into an atom and creates a fast-moving muon.
The February flash was thirty-five times brighter than any previous sighting, earning it the nickname “impossible muon.”
However, a puzzle soon surfaced. IceCube has more than 10 years of data, a much larger collection area, and a clear view of the same part of the sky. If a cosmic engine was sending such strong neutrinos toward Earth, IceCube should have seen them first—but it had none that matched.
This mismatch bothered some researchers. They began to ask whether the muon had come from something even stranger than a neutrino.
In a new study, the team argues that the flash could be the first sign of dark matter ever detected on Earth. Dark matter, an invisible form of matter that outweighs the normal kind by a factor of five, has revealed itself only indirectly through gravity so far. Many experiments have tried to trap it directly, but none have done so.
A Strange Cosmic Particle Lit Up the Sea
The group’s idea starts in a type of galaxy called a blazar. A blazar contains a supermassive black hole that launches narrow, powerful jets of particles at nearly the speed of light. If those jets include a special kind of dark matter particle, the beam could travel for billions of years and still remain intact. The path traced by the February flash points back to a region of sky that hosts several known blazars, so the setting fits.
Because the beam arrives at almost the same level as the ground—not straight down—it slips sideways through Earth’s outer layer instead of plunging toward the planet’s center. To reach the KM3NeT detector in the Mediterranean, each particle has to thread its way through about 93 miles (150 kilometres) of rock and soil along that shallow route.
While making this long underground trek, a dark-matter particle can strike an atomic nucleus much like a cue ball hitting the eight-ball. The collision jolts the dark matter particle into a short-lived “excited” version of itself that is heavier than before. This excited state lasts for only a blink of time—far less than a millionth of a second—before it breaks apart into two muons. Because those muons leave the split almost perfectly aligned, they fly side by side down the same corridor of seawater. KM3NeT’s light sensors, which detect the faint blue glow the muons create, cannot separate the twin paths; they record a single extra-bright streak, which is exactly what scientists saw in February.
IceCube’s story is different because of its location. From the South Pole, the same beam would pass through only about nine miles (15 kilometers) of crust before reaching the detector. With far less rock in the way, the chance for dark matter to collide and show itself drops sharply. That simple depth difference can explain why KM3NeT saw a brilliant flash while IceCube stayed quiet.
P. S. Bhupal Dev of Washington University in St. Louis told New Scientist that the result “opens up a new way you can really test dark matter.” He notes that with the right conditions, “we can convert these neutrino telescopes into dark-matter detectors.”
The idea thrills some researchers because it would give science a fresh tool in the long hunt for the universe’s missing mass.
Other experts advise patience.
“(T)his is probably just an ordinary neutrino that’s exceptional in energy,” Dan Hooper of the University of Wisconsin–Madison told New Scientist.
In his view, the simplest answer remains a lone neutrino that happened to reach record-breaking energy.
Shirley Li at the University of California, Irvine, adds another check. The dark-matter picture predicts two overlapping muons, not one, but today’s instruments cannot yet tell the difference at such extreme energies. She said it is “potentially testable,” yet reconstructing a double track at this brightness remains very hard.
The debate will not stay open for long. KM3NeT is still under construction and will add more light sensors over the next few years. IceCube continues to watch the same patch of sky, and a planned upgrade will sharpen its vision. If the Mediterranean telescope logs more giant flashes from the same direction while IceCube remains quiet, confidence in the dark-matter idea will grow. If both detectors begin seeing similar events, the explanation may circle back to extra-energetic neutrinos or another unknown cause.
Either result would matter. A second event would mean that cosmic accelerators can drive particles to mind-bending energies, no matter what those particles are. A confirmed dark-matter hit would be even bigger, giving researchers their first direct clues about the stuff that binds galaxies together. For now, one flash beneath the Mediterranean has reopened one of physics’ largest mysteries and given scientists worldwide a brand-new trail to follow.
The findings were reported in the preprint server arXiv.
