Armstrong stepped into history July 20, 1969, leaving the first human footprint on the surface of the moon. NASA/Getty Images hide caption

itoggle caption NASA/Getty Images

A tenuous thread of dark matter is seen connecting the galaxy clusters Abell 222 and 223. Courtesy Jörg Dietrich/Universitäts-Sternwarte München hide caption

itoggle caption Courtesy Jörg Dietrich/Universitäts-Sternwarte München