AUSTIN, Texas – Tipping the scales at two quadrillion times the mass of the sun, the El Gordo galaxy cluster is the largest, hottest, and most energetic cluster ever seen.

Officially called ACT-CL J0102-4915, astronomers nicknamed the cluster "El Gordo" – meaning the fat one – due to its heft. A large portion of its mass is in the form of dark matter, an invisible material that pervades the universe.

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.

Because dark matter hardly interacts, even with itself, the dark matter halos of the clusters are thought to be essentially collisionless, said Hughes. "They stream through each other," he said.

It is only the gas and dust of the clusters and interacts, producing tremendous shocks and releasing large amounts of energy.

Image: X-ray: NASA/CXC/Rutgers/J.Hughes et al, Optical: ESO/VLT/Pontificia Universidad. Catolica de Chile/L.Infante & SOAR (MSU/NOAO/UNC/CNPq-Brazil)/Rutgers/F.Menanteau, IR: NASA/JPL/Rutgers/F.Menanteau