Caption: The massive galaxy cluster RXJ2248-4431 at a redshift of 0.35 (nearly three billion light years distant) in this 8' x 8' closeup image contains over 1,000 galaxies bound together gravitationally with a total mass of some 100 trillion times that of the Sun. These are the largest known bound structures in the visible universe. Only a few percent of the mass is in the form of visible starlight (the small yellowish blobs) -- the rest is in hot X-ray gas and invisible dark matter. The dark matter component makes its presence known though the phenomenon of strong gravitational lensing -- manifest here as bluish arcs -- which are themselves the elongated images of more or less circular blue background galaxies at more than twice the distance of the cluster itself. The Dark Energy Survey is producing the largest Southern hemisphere optical catalog of galaxy clusters made to date.