Dark matter is an invisible substance that reveals itself only through gravitational effects: it bends the light of distant galaxies, much like an unseen window that alters the path of light rays.
In 1998, physicists caught neutrinos doing something astonishing: the particle changes its type on the fly. That is only possible if it has mass. This became the only laboratory proof of physics beyond the Standard Model. But exactly how much a neutrino weighs remains a mystery.