This Hubble Space Telescope composite image shows a ghostly ring of dark matter in the galaxy cluster Cl 0024+17. The ring-like structure is evident in the blue map of the cluster's dark matter distribution. The map is superimposed on a Hubble image of the cluster. The ring is one of the strongest pieces of evidence to date for the existence of dark matter, an unknown substance that pervades the universe. The map was derived from Hubble observations of how the gravity of the cluster distorts the light of more distant galaxies, an optical illusion called gravitational lensing.