This gently smiling cosmic face was released in November 2015 to celebrate the 100th anniversary of Albert Einstein’s theory of general relativity. It is a composite image of a cluster of galaxies seen in visible light by the Hubble Space Telescope and in X-rays (purple) by NASA’s Chandra X-ray Observatory. The cluster has been nicknamed the "Cheshire Cat" because of its resemblance to a smiling feline. Some of the arched features are actually distant galaxies whose light has been stretched and bent via the distortion of space–time by the cluster’s mass, which is mostly in the form of dark matter. This so-called strong-gravitational-lensing effect was predicted by Einstein, but he thought it would never be observable. Astronomers estimate that the two "eyes" of the cat will merge in about one-billion years, leaving one giant elliptical galaxy at the centre. The cat will then become a smiling Cyclops.