QSO 0957+561 was the first object ever to be identified as a gravitational lens in space. It’s a single quasar, located some 8 billion light-years from Earth. But it looks like two quasars, due to the presence of an gigantic mass located in space between Earth and the quasar. The intervening mass – a distant galaxy – bends light from the quasar, so that we see two quasar images instead of one.
http://www.earthsky.org/radioshows/48829/dark-matter-through-a-gravity-lens |