A giant tank of ultrapure water sits deep underground in Japan. Its walls are lined with thousands of light-sensitive sensors that catch faint flashes — the traces of ghostly particles passing through the water. This detector, Super-Kamiokande, has long been staring at the sky for the neutrino echo of ancient stellar explosions. And then — an unexpected blip. At an energy of about 20 MeV, slightly more events were recorded than calculations predicted. The extra signals are weak, but the statistics say it's not just random instrument noise.
So where does the excess come from? The authors of a new study offer a bold explanation: perhaps the detector caught not the echoes of supernovae, but the 'breath' of dark matter. According to the hypothesis, dark matter particles collide with each other and turn directly into neutrinos — light, almost undetectable particles. To explain the signal, dark matter would have to be relatively light — with a mass of tens of MeV (millions of electron volts). For comparison: the electron's mass is about 0.5 MeV, so the candidate particle would be about a hundred times heavier than an electron, but still thousands of times lighter than a proton. Such a mass arises naturally in extensions of the Standard Model — the set of rules describing known particles and forces.
If the guess is right, this would be the first real hint at the nature of dark matter — not through its gravitational pull, but through direct particle production. Other neutrino telescopes, such as the under-construction JUNO in China, could verify this. Their sensitivity should be enough to either confirm the signal or rule out the hypothesis. Meanwhile, the giant detector keeps peering into the depths of the water, waiting for new flashes that might reveal what the invisible part of the universe is actually made of.
The water in the Super-Kamiokande detector is so pure that if you dove in with a flashlight, you wouldn't see the beam — light only scatters off particles, and there are almost none there.