How "fuzzy" can dark matter be? A gravitational lens provides the answer

Dark matter, which makes up more than 80 percent of the mass in the universe, neither absorbs nor emits light and interacts with light and normal (baryonic) matter only through its gravity. The nature of dark matter is one of the most important open questions in astrophysics and cosmology. A theoretical model for dark matter, called "Fuzzy Dark Matter" (FDM), imprints a characteristic signature on the light that is bent around a massive galaxy (a so-called gravitational lens). By analyzing a gravitational lens system observed in the radio range with extremely high angular resolution, we have determined how "fuzzy" dark matter can be.