Abell 1656 is a large cluster of galaxies found in the direction of the constellation Coma Berenices, at over 300 million light years from us. It is known to have over 1000 galaxies in it, about 500 of which are visible in this image of mine.
This cluster is well known for being the first one to which Fritz Zwicky applied, in 1933, the virial theorem, finding that the visible matter wasn’t enough to explain for the velocities being observed, and ushering in the idea of some invisible "dark matter", necessary to explain the observed gravitational effects.
Full acquisition data, high-res imagery and annotated version (a must see!) are at the Astrobin page. Comments and critique are very welcome.