A team led by Vasily Belokurov at the University of Cambridge has found the largest and most complete optical Einstein ring known to date. The object, which was dubbed the „Cosmic Horseshoe“, provides a unique laboratory for studying what the universe was like at less than a quarter of its present age.
An Einstein ring is a kind of cosmic mirage. Gravity from a foreground galaxy distorts light from a background galaxy into an arc. If the alignment between the background galaxy and the foreground „lensing“ galaxy is precise enough, the distant galaxy’s light becomes warped into a complete ring.

https://arxiv.org/abs/0706.2326v3