Gravitational lensing happens when the light from a distant object passes close to something massive, like a galaxy or a cluster of galaxies, on its way to us. Because mass curves spacetime, the light’s path bends around that massive object the same way light bends passing through a glass lens.
Depending on the alignment, this bending can magnify a distant galaxy, smear it into a curved arc, or even split a single object into multiple visible images. In extreme cases of near perfect alignment, it produces a full ring of light called an Einstein ring, wrapped completely around the foreground object.
Gravitational lensing gave astronomers one of the earliest confirmations of general relativity, when starlight bending around the sun during a 1919 solar eclipse matched Einstein’s predictions. Today it is a working tool used to map invisible dark matter and to study galaxies too faint to see any other way.