Einstein Cross: Gravity bends light so that two galaxies look like 5!
Relativity predicts that gravity warps the shape of space and time so that light will no longer move along a straight path. One of the earliest confirmations of the theory was during a total eclipse which allowed us to see a star that wasn't quite in the right place. The sun's gravitational force bent the light rays so that the star appeared about an inch away from where it actually was.
In the image above, the galaxy in the center bends the light waves coming off of the object behind it so that we see 4 different images of the same object. This is called gravitational lensing.