Gravitational lensing occurs when a massive object bends light. It was one of the first predictions of Einstein’s theory of general relativity. It takes a lot of gravity to make gravitational lensing visible. One type, weak lensing, where the bending is caused by the light of a distant galaxy passes through a cluster of galaxies. That makes a slight bend. But there’s another type that is much more dramatic, and much more rare. Here’s more form University Today:

The second type is strong lensing, and it is rarer. For strong lensing, a distant galaxy has to be nearly blocked by a closer one. In this case, the light of the distant galaxy is strongly distorted, often into arcs of light surrounding the closer galaxy. Since the amount of distortion depends on the mass of the closer galaxy, it lets us measure the amount of dark matter in the galaxy. It also allows us to measure the expansion rate of the universe…. Recently a team used an AI program to find lensed galaxies in sky survey data. After training the program on known lensed and unlensed galaxies, it then combed through observations to find more than 300 candidates lensed galaxies. Many of them were then confirmed by follow-up observations with the Hubble Space Telescope.

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