Transcript: The most massive bound objects in the universe, clusters of galaxies, can also deflect light. Lensing by clusters produces the interesting phenomenon of multiple images of background galaxies along with distorted images of the galaxies where they form little arcs of light. Cluster lensing has now been observed in dozens of cases. It’s best seen with the Hubble Space Telescope. With its extraordinarily sharp images, the tiny little arcs are easily visible. Typically a relatively nearby cluster of galaxies at a redshift of a few tenths, 0.2 to maybe 0.5, and consisting mostly of massive red elliptical galaxies will cause distortion and multiple images of background distant galaxies that are often blue and are at a redshift of one or two. Many pairs of images or many arcs are seen in lensing clusters, and these little images can be used to reconstruct the mass distribution of the cluster. This analysis confirms that clusters of galaxies are overwhelmingly composed of dark matter.