General relativity is Einstein's theory of gravitation that builds on the geometric concepts of space-time introduced by special relativity. Einstein was looking for a more fundamental explanation of gravity than the empirical laws of Newton. Besides coming up with a different way of thinking about gravity (in terms of geometry), general relativity makes a series of specific predictions of observable deviations from Newtonian gravitation, especially under strong gravitational fields. These predictions provide a stringent test of Einstein's theory..

In this image of a cluster of galaxies, the light of an even more distant galaxy is bent into an arc (Einstein ring) by the severe curving of the geometry of space-time.

This curvature is caused by the large mass of an intervening galaxy.