A galaxy cluster's gravity warps spacetime, acting like a colossal lens: it focuses and amplifies light from distant objects, as predicted by Einstein. Such lenses are rare, but they let us glimpse what lurks at the edges of the cosmos.

The most unexpected find: one lens gave itself away with a microscopic arc that was previously dismissed as an image defect. Now these lenses help astrophysicists: by analyzing the light bending, they calculate how much invisible dark matter hides in the clusters, and how dark energy accelerates the Universe's expansion. The data is open to all observatories worldwide.