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.
Astronomers distinguish between strong lensing, dramatic enough to produce visible arcs, rings, or multiple images, and weak lensing, a far subtler distortion that only slightly stretches the shapes of background galaxies and has to be measured statistically across huge numbers of them. There is also microlensing, a brief brightening of a background star’s light as a smaller foreground object, sometimes even a rogue, unbound planet, passes almost exactly in front of it. Because microlensing does not depend on a planet emitting or reflecting any light at all, it has become a valuable technique for finding planets that would otherwise be completely undetectable, including free floating planets that were ejected from their home star system and now drift through the galaxy entirely alone.
Fritz Zwicky proposed as early as 1937 that gravitational lensing by galaxy clusters could be used to weigh those clusters directly, decades before technology existed sensitive enough to actually observe the effect clearly. That idea proved prophetic, today lensing is one of the primary tools cosmologists use to map dark matter’s distribution across the universe, since it responds to all mass, seen or unseen, rather than only to the light a galaxy happens to give off.