Gravitational lensing is the process by which light travelling from a distant source seems to "bend" or curve around extremely dense objects that lie between the light source and the observer. It is an extremely important aspect of modern astronomy because the gravity of a dense object can act as a sort of cosmic lens, allowing astronomers to focus on objects that lie beyond it. Gravitational lensing, as Time magazine reported in 2012, is helping astronomers "redraw the map of space."
The existence of gravitational lenses was predicted by Alfred Einstein in his general theory of relativity. The space-time around a massive object, according to Einstein, was curved or distorted by its gravity. Light passing through this distorted region of space-time would thus be diverted itself, appearing to bend or curve. At the time, the telescopes (and computers) necessary to conduct a systematic search of the sky for distant gravitational lensing did not exist. The first such object was only discovered in the 1970s by American astronomers at Kitt Peak National Observatory.
Now that numerous gravitational lenses have been identified, the process is fairly well understood, and has been explained to the public by NASA. In most cases, stars and galaxies are visible in the night sky because there is nothing between them and Earth: The light they emit travels more or less directly to the telescope or the human eye. Sometimes, however, a second extremely dense object - like another galaxy, or a black hole - lies directly on the path between Earth and an even more distant galaxy. Light from this second, more distant, source gets pulled off course as it travels by the massive object in between, so that when it reaches human eyes, it's no longer travelling on the vector or direction in which it was first emitted.
The result might only be a minor distortion, or, depending upon the relative location of the light source and the gravitational lens, as well as the gravitational strength of the lens, it could produce strange optical effects - the distant object appearing as a sort of ring around the closer object, or even being fragmented into multiple partial images around the object, like Einstein's cross. For instance, the first lens to be discovered in the 1970s, called the Twin Quasar, is now known to be an active galactic nucleus about 8.7 billion light-years away from Earth, the light from which is wrapping itself around a second, giant elliptical galaxy which lies directly in between the "Twin Quasar" and Earth, at a distance of about 3.7 billion light-years. The closer elliptical galaxy causes the distant quasar to appear as two virtually identical objects in close proximity when viewed from Earth.
Einstein initially considered gravitational lensing as a sort of optical quirk that, if it could be found, would be evidence for his theory of relativity. Today, physicists are no longer looking for evidence to confirm the theory of relativity. Instead, they're now using the existence of gravitational lenses to help them search for other more exotic deep-space phenomena and to test radical new theories. For instance, scientists hope that the search for new gravitational lenses will lead them to evidence of an even more elusive and mysterious substance predicted by modern physics, known as dark matter.