One dramatic prediction of General Relativity is that gravity bends the path followed by a light signal. In cosmology this leads to the phenomenon of gravitational lensing.
Suppose that light from a very distant bright object that we observe, for example a quasar, passes through a compact cluster of galaxies. The strong gravitational field of the cluster will bend the light, thereby distorting the image into an arc or a circle (also called an Einstein ring), or possibly creating multiple images (for example, the Einstein cross, a four-fold image of a distant quasar). In this way the cluster of galaxies acts as a gravitational lens.
Click on the images for an enlarged view.
Images of Einstein rings
Images taken by the Hubble Space Telescope.