Gravitational mirages are a phenoma that happen at such a large scale that it’s difficult for us humans to imagine them. However we know they exist. This happens in galaxy clusters, which happen to be one of the most massive and large objects known by mankind. On this HST picture figures one of the largest galaxy clusters that is known today: SDSS J1004+4112. So what is a gravitational lens?
Check out the following picture:

You can see 5 quasars: these are actually one quasar. There is an explanation to that effect. This cluster’s mass must weigh about a million billion billion solar masses. This mass is so large that it deviates light waves from their normal course.
Because this cluster deviates light, it is as if we had placed a gigantic magnifying glass in front of us, wich can be used as…a telescope! A telescope 20 million light years across. How big is that? Well, imagine light, which is the fastest thing in the universe, emitted at one end of the object. It would take 20 million years to get from one end to the other. How cool do we feel with our super 10m telescopes? What if we used our own instruments to create a double telescope with the cluster? This is how we discovered some of the most distant and faint objects known today. Astronomers observe objects behind clusters. The universe did grant us a few presents like these. This is one of the ways we can date the universe: about 13,7 billion years old.