We can use General Relativity to look for the presence of very massive, compact objects. Another prediction of Einstein's General Relativity is that a very massive compact object, like a black hole, directly in between us and a distant galaxy will cause the light of the distant galaxy to be appear broken up and spread out in a circle, much the way certain glass lenses distort light. If the galaxy is perfectly lined up with the black hole, then we should see the galaxy's light spread out into a perfect circle. If the line-up isn't exactly perfect, the galaxy's light should appear broken up into parts of a circle. This predicted phenomenon is called gravitational lensing. The expected shapes came to be known as "Einstein rings" long before they were observed.