Neutrinos are the most abundant massive particles in the universe. Trillions of them pass through us every second, yet they leave barely a trace. Their collective gravity, though tiny, can influence the motion of entire galaxies. Think of two people approaching each other on a busy street; invisible passersby alter their speed. Similarly, for galaxy pairs, neutrinos act as those unseen passersby, subtly distorting their motion. Comparing with computer simulations yielded a total mass of about 0.24 electronvolts and, more surprisingly, exposed an imbalance: more neutrinos than antineutrinos. This skew contradicts the simplest versions of the Standard Model and calls for new physics.
Such an excess of neutrinos in the early universe could have been a hidden factor steering the growth of galaxy clusters. The method opens a way to study neutrino properties without giant accelerators – simply by watching the dance of galaxies.
A single neutrino is a million times lighter than an electron, but there are a billion times more neutrinos than atoms in the universe.