Gravitational lens model.

A large mass such as a dark matter cluster, a black hole, a galaxy or group of galaxies can distort spacetime much like a lens so that light from a distant source can be bent towards the viewer, creating distorted multiple images of the source. In this demo, a clear plastic lens has been shaped to simulate the kind of distortion created by a large mass. If the lens is moved over a piece of paper with a black dot on it, the dot can appear to be two distorted spots, or even a complete ring called an Einstein ring.