Einstein published his theory of general relativity in 1916 and to this date it's the theory that best describes the phenomenon known as gravity. A large consequence of general relativity is the outcome that gravity is no longer a force as Newton would have us believe, but rather, because space and time are connected as a continuum called spacetime, gravity is the observed result of inertial motion in spacetime that is curved by matter and radiation.
One interesting phenomenon that general relativity also explains is gravitational lensing. When a very massive object, a cluster of galaxies for example, exists somewhere in space it will curve spacetime very noticeably. Gravitational lensing is what happens when that galaxy cluster gets in between us (here on Earth) and very distant galaxies or quasars even further away. Light gets emitted from these distant galaxies and quasars in all directions. The curvature of spacetime, however, effects even photons, which are massless but have momentum. Thus, the gravitational lens that is the curved spacetime flings photons our way, when they were flying off towards some other distant point in space. The result of this redirection is that astronomers will observe odd ornaments around a galaxy cluster. One single distant point source object like a quasar will be found in multiple places, while large galaxies in the background will be distorted as arcs. One of my favorite cosmic images of all time is a gravitational lens.
In this image from APOD posted on my birthday a while ago, a galactic cluster has gravitationally lensed a quasar and its host galaxy. This particular quasar can be seen five times around the cluster. Another much larger yellowish galaxy can be seen arced twice on the right side of the cluster.
What is arguably the most amazing thing about gravitational lensing is how it can be used to map dark matter. Through a process that is way beyond my current understanding, astronomers can survey cosmic objects many times and measure how much gravity is lensing their light towards us. With these measurements, of how spacetime gets curved, it is possible to map out the distribution of all matter in the area, including dark matter. Some examples of these maps are shown below.
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Mass contours of the Bullet cluster
Large-scale distribution of dark matter made from Hubble Data
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