... Explanation: How do you calibrate a huge gravitational lens? In this case the lens is the galaxy cluster Abell 383 , a massive conglomeration of galaxies, hot gas, and dark matter that lies about 2.5 billion light years away ( redshift z=0.187). ... This technique compl e ments other measures including computing the dark matter needed to contain internal galaxy motions , to confine cluster hot gas , and to create the gravitational lens image distortions. ... The Hercules Cluster of Galaxies . ...