Gravitational lensing
Strong lensing betrays itself by producing multiple images from the same object. A well-known example is Einstein's Cross (Q2237+0305) at a distance of 8 billion lightyears. Here, the gravitational lens produces four images from the same object (a quasar), although it is only one object. The light of the quasar does not travel in a straight line to earth. Instead, it is bent along the gravitational field of a galaxy in front of it. This galaxy is at a distance of 400 million lightyears.