Dark matter, which makes up over 80% of the mass in the Universe, does not absorb or emit light, interacting with light and normal (baryonic) matter only through its gravitational pull. The nature of dark matter is one of the major open questions in astrophysics and cosmology. One theoretical model for dark matter, known as fuzzy dark matter (FDM), is predicted to leave a very specific imprint on light that is bent around a massive galaxy in a phenomenon called gravitational lensing. By examining the radio light in a gravitational lens system observed at extremely high angular resolution, we have determined just how “fuzzy” the dark matter can be.
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