Something doesn’t look quite right in clusters of galaxies. The structure of the dark matter that suffuses these enormous clumps of matter is far different to what we would expect, and none of our current models can explain this behaviour.

One way to measure dark matter in distant galaxies or clusters of galaxies is by analysing how they affect the light coming from objects behind them. When light passes a massive object, such as a galaxy full of dark matter, the gravity of …