This composite image of a ghostly ring of dark matter in the galaxy cluster Cl 0024+17 was taken by the NASA/ESA Hubble Space Telescope in November 2004. A false color map is superimposed on a Hubble image of the cluster in order to make the cluster’s dark matter distribution more evident.
Of course dark matter cannot be seen directly, but its presence can be inferred by measuring how the gravity of the cluster Cl 0024+17 distorts the light of more distant galaxies located behind it from Earth’s point of view. This optical illusion is called gravitational lensing.
It has been suggested by astronomers that the dark matter ring was formed when two gigantic clusters collided and merged together to form Cl 0024+17.