Faint Light Reveals the Shape of Invisible Dark Matter ✎ express
In galaxy clusters, invisible dark matter serves as the framework holding star systems together. Its shape is betrayed by a faint glow — stars ripped from galaxies by tidal forces. This stellar 'broth' fills the cluster like a liquid taking the shape of its container: it envelops the clumps of dark matter, creating a precise cast of their outlines. Astronomers measured the elongation of the light cloud and compared it to the shape of the dark halo: the direction of flattening matches to within ten degrees, and the contours themselves differ by only a few percent. Astonishingly, this glow is made of billions of stars, yet together they shine fainter than a large galaxy, less than one percent of its brightness. The method uses photometry — a precise measurement of brightness that reveals faint details. It provides a more reliable picture than previous estimates based on galaxy motions. Back in the 1930s, Fritz Zwicky suspected invisible mass in clusters; now its outlines can be read in the silent light.
Though billions of stars produce this light, altogether they shine at less than 1% the brightness of the cluster's central galaxy.