A direct consequence of Einstein’s theory of general relativity, and an observational way to prove it, is gravitational lensing.  It requires a powerful gravitational source to work, such as a galaxy or cluster of galaxies.  It works in a similar way to a lens of glass, where rays of light are bent toward a single source, increasing the brightness.  In this case, instead of glass, the bending of the rays is due to the curvature of space. Light rays coming from the source would otherwise miss Earth, but instead are bent toward us when there is a massive object in front of it.  It’s...