Gazing into the universe is like looking into a funhouse mirror. That's because gravity warps the fabric of space, creating optical illusions.
Many of these optical illusions appear when a distant galaxy's light is magnified, stretched, and brightened as it passes through a massive galaxy or galaxy cluster in front of it.
This phenomenon, called gravitational lensing, produces multiple, stretched, and brightened images of the background galaxy.
This phenomenon allows astronomers to study galaxies so distant they cannot be seen other than by the effects of gravitational lensing.
Source
Many of these optical illusions appear when a distant galaxy's light is magnified, stretched, and brightened as it passes through a massive galaxy or galaxy cluster in front of it.
This phenomenon, called gravitational lensing, produces multiple, stretched, and brightened images of the background galaxy.
This phenomenon allows astronomers to study galaxies so distant they cannot be seen other than by the effects of gravitational lensing.
Source