You're looking at ACT-CL J0102-4915. To it's stargazing buddies though, it's better known as El Gordo — literally, "the fat one" — because it's the largest known galaxy cluster in the distant universe.
Researchers have worked that out by measuring how much the cluster's gravity warps images of galaxies in the distant background. The latest results show that it weighs three million billion times as much as our sun — 43 per cent more massive than earlier estimates. This image, which was used in the analysis, was captured by Hubble. [NASA]