Galaxies Collide

Wired:

Galaxy clusters are the largest known objects in the universe, occurring when hundreds or even thousands of galaxies come together.
El Gordo is located more than 7 billion light years from Earth. At this distance, the universe was only half its current age, presenting a puzzle for researchers. Could such a massive cluster have formed so early in the universe?
“Although El Gordo is a very rare object, it’s not inconsistent with current formation theories,” said astronomer Jack Hughes of Rutgers University, who presented the giant object Jan. 10 at the American Astronomical Society meeting.

Part of the reason for the cluster’s enormous size is that it is was once two separate clusters that are now undergoing a collision. El Gordo has two density peaks, corresponding to the centers of the fuzzy purple blobs in the image above, indicating the locations of the two clusters.
The bluish feature in the center of the picture is a large gas and dust pocket caught at the point of the crash. The wake of this violent impact appears as a hazy tail streaming toward the upper right in the image.