Imagine that you are in orbit around a black hole at a safe distance outside the event horizon. What would the sky look like? Normally you would just see the
background stars steadily sliding by, due to your own orbital motion. But the gravitational force of a black hole changes things considerably.
Light rays that pass close to the black hole get caught and cannot escape. Therefore, the region around the black hole is a dark disk. Light rays that pass
a little further away don't get caught but do get bent by the black hole's gravity. This makes the starfield appear distorted, as in a funhouse mirror. It also
produces multiple images. You would see two duplicate images of the same star on opposite sides of the black hole, because light rays passing the black hole on
either side get bent toward you. In fact, there are infinitely many images of each star, corresponding to light rays that circle the black hole several times
before coming toward you.
Einstein's theory of general relativity predicts that every object bends light rays through its gravity. This is called gravitational lensing. For our Sun
this effect is very weak, but it has been measured. For more massive and distant objects in the Universe much stronger lensing has been seen. However, it has not
yet been possible to observe this effect near a black hole, or to directly photograph the dark disk surrounding a black hole. However, this may become possible
in the foreseeable future.
The gravity of this galaxy cluster (called Abell 2218) bends the light from more distant galaxies behind it, creating thin arcs of light.