Deep underground in Japan, a huge tank of ultrapure water waits for rare visitors — neutrinos arriving from distant stellar explosions. Such guests are few and far between: over years of observation, Super-Kamiokande registers only a handful of events. But recently, physicists noticed that the guests are slightly more numerous than expected.
This excess is like a quiet ring in the night — faint but persistent. Usually, such signals are attributed to the diffuse supernova neutrino background (the neutrino echo of all stars that have exploded throughout cosmic history). However, calculations show that explosions cannot produce that many events. That's where a bold idea comes in — maybe the ring isn't from a star, but from dark matter itself?
The authors build a model: a dark matter particle with a mass of about 22 MeV (slightly heavier than an atomic nucleus, but hundreds of times lighter than a proton) collides with its antiparticle and turns into two neutrinos. These neutrinos, traveling through space, partially change their type — converting from muon and tau neutrinos into electron antineutrinos. It's these that the detector sees as extra signals. This mechanism not only explains the observations but also predicts how much dark matter should have remained after the Big Bang. And that amount matches what astronomers see in the motion of galaxies.
Of course, the hypothesis needs testing. Other experiments (Borexino in Italy, accelerator searches at the NA64-μ facility) impose strict constraints: the particle must be very careful not to reveal itself too early. So far, it fits within these bounds. If future data confirm the guess, we'll have caught dark matter for the first time not by its gravitational trail but by direct contact — through a faint neutrino whisper from deep within the mountain.
The Super-Kamiokande detector contains 50,000 tons of ultrapure water: that's about 20 Olympic swimming pools. The water is so clear that a beam of light would travel 70 meters through it before its intensity halves.
In Stanisław Lem's novel 'Solaris', an ocean communicates with humans through elusive signals — much like dark matter might 'speak' to us through rare neutrino events.