... Intervening dark matter , which is normally invisible, might show its presence by distorting images originating in the distant universe , much the way an old window distorts images originating on the other side. By noting the degree to which background galaxies appear unusually flat and unusually similar to neighbors, the dark matter distribution producing these weak gravitational lensing distortions can be estimated. ... Elliptical Galaxy NGC 1132 . Abell 2218: A Galaxy Cluster Lens . ...