For decades, the standard model of particle physics held that neutrinos—tiny, nearly massless particles produced in nuclear reactions—had no mass at all. However, this model was challenged by the 'solar neutrino problem': detectors on Earth were recording only a fraction of the neutrinos predicted to be coming from the sun. The solution came with the discovery of neutrino oscillations. Neutrinos exist in three types, or 'flavors.' As they travel through space, they can spontaneously transform from one flavor into another. Because the early detectors were only sensitive to one flavor, they were missing the neutrinos that had switched during their journey. For this oscillation to occur, quantum mechanics requires that neutrinos must possess a non-zero mass. This discovery forced a significant revision of the standard model and opened new questions about the nature of dark matter, as neutrinos' mass, though tiny, could have significant gravitational effects across the universe.