5 billion light-years
The combined gravity of a cluster of galaxies can act as a cosmic lens. In this Hubble Space Telescope image, a rich cluster (yellow) bends the light from more distant galaxies, leaving them as blue streaked arcs. Studying the shapes and positions of these images, astronomers find there isn't enough visible matter to account for the distortions, so there must be a large amount of invisible dark matter present.
Credit: Image made in visible light by M. Lee and H. Ford for NASA / ESA / JHU.