Ethan Siegel in Forbes:
In order to test General Relativity as a theory of gravity, you need to find a system where the signal you’ll see differs from other theories of gravity. This must at least include Newton’s theory, but should, ideally, include alternative theories of gravity that make distinct predictions from Einstein’s. Classically, the first such test that did this was right at the edge of the Sun: where gravity is strongest in our Solar System.
As light from a distant star passes close to the limb of the Sun, it should bend by a very specific amount, as dictated by Einstein’s theory. The amount is twice that of Newton’s theory, and was verified during the total solar eclipse of 1919. Since then, a number of additional tests have been performed to great precision. Each and every time, Einstein’s theory has been validated, and alternatives emerge defeated. Yet on scales larger than the Solar System, the results have always been inconclusive.
Until today. We’ve finally taken that first step towards verifying General Relativity on those large, cosmic scales, where gravity is often the only force that matters.