Neutrinos — elementary particles rightly dubbed “ghosts”—barely interact with matter, making their detection a formidable challenge. The existence of neutrinos was predicted by Wolfgang Pauli, and the name was coined by Enrico Fermi. These particles are born in extreme astrophysical processes, such as supernova explosions or near black holes in our galaxy, and pass through planets and stars without hindrance. To catch them, the Baikal-GVD neutrino telescope has been deployed in the waters of Lake Baikal. The detection principle is this: when a neutrino does collide with an atom of water, it produces a charged particle, a muon, that, moving through water faster than light does in that medium, generates a faint flash of Cherenkov radiation. The telescope's sensitive optical sensors pick up even single photons of this “glow,” allowing scientists to reconstruct the direction and energy of the original neutrino.

Over five years of observations, astrophysicists analyzed 92 patches of the celestial sphere suspected of harboring high-energy neutrino sources. The main event-selection criterion was the arrival direction of muons: they looked for those moving upward, such muons could only have been produced by neutrinos that had traversed the entire Earth, which filtered out background particles from the atmosphere. No bright, statistically significant signals were found; however, the researchers determined upper limits on the neutrino fluxes from each object: that is, how many neutrinos definitely do not arrive. The only weak hint of an excess over background came from the massive star cluster Westerlund 1. Such a null result is not a failure but a valuable tool for astrophysics: knowing where ghostly particles are absent narrows the search area and boosts the chances of spotting those still hiding in the data.