Spacetime is curved. We’ve all heard the line. But what does it mean? Well on the largest scales, the curvature of spacetime is abundantly clear as the warped fabric of the universe distorts images of distant objects.
The image below is of the Abell 2218 galaxy cluster, taken by the Hubble Space Telescope. The cluster is very massive so it warps the spacetime around it. This warped spacetime acts as a lens so that light light coming from galaxies behind Abell 2218 is spread out much more than it should be. The result is that images of galaxies behind Abell are very distorted. In the most extreme cases, the galaxy becomes the rings you see in the image, called Einstein rings.
Astronomers use gravitational lensing to view distant objects they wouldn’t otherwise be able to, and to search for objects that are invisible except by the lensing effect they produce on the starscape behind them, such as dark matter.
The gravitational lensing of light by our own sun was also the first major confirmation of the validity of general relativity.