According to general relativity even light is affected by the mass of an object. In fact, when a ray of light passes near a large mass, its path is diverted. This shift is known as the gravitational lens and was one of the first confirmed effects of Einstein's theory.
There are two types of gravitational lenses and both play an important role when studying the understanding and evolution of the universe. The first is a "weak" lens, in which light from a distant galaxy passes through a cluster of galaxies, but not near a particular galaxy. Therefore, the bending of the light is small. By observing these distortions, astronomers can measure the average density of matter in the universe, helping us understand dark energy as well.
The second type is a "strong" lens, and it is rarer. For this to happen, a distant galaxy must be almost blocked by a closer one. In this case, the light from the distant galaxy is heavily distorted, often in arcs of light surrounding the nearest galaxy. Since the amount of distortion depends on the mass of the nearest galaxy, it allows us to measure the amount of dark matter in the galaxy and also the rate of expansion of the universe.
It is therefore logical to think that "strong" lenses are very important. Fortunately we are starting to find more and more. Recently a team used an artificial intelligence driven program to find galaxies with these lenses; have been found more than 300 candidates. Many of them confirmed by follow-up observations with the Hubble Space Telescope.
The next goal will be to analyze other sky data to find – at least – a thousand galaxies with strong gravitational lenses.