Astronomers have glimpsed the largest cluster of galaxies ever seen in the distant, early universe.

The discovery of this far-off group, estimated to contain as much mass as a thousand large galaxies, offers further proof of the existence of the enigmatic force called dark energy.

"This is the most luminous, and therefore probably the most massive, cluster of galaxies discovered at this epoch," said Georg Lamer of the Astrophysikalisches Institut Potsdam in Germany, who led the team that discovered it. "The light we observe started about 7.7 billion years ago. This is about half of the age of the universe, so it is from quite long ago, and quite far away."

When astronomers look at distant objects, they are looking back in time, in this case seeing objects that are 7.7 billion light-years away.

Lamer and his team discovered the cluster, known by its catalogue number, 2XMM J083026+524133, by chance while they were surveying a portion of sky for a catalogue of X-ray sources. Using the European Space Agency's orbiting X-ray observatory XMM-Newton, they spotted an extremely bright object without any galaxy visible in optical light nearby.

After noticing the aberrant object, they took a deeper exposure with the Large Binocular Telescope in Arizona and determined that the source of the light was a far-off group of galaxies containing the mass of about 1,000 Milky Ways.

The astronomers say the discovery offers further proof of the mysterious force called dark energy that scientists think propels the acceleration of the universe's expansion. Dark energy is believed to account for about 70 percent of the universe, with the remaining portion made up of normal matter and its enigmatic sister, dark matter.

"The existence of the cluster can only be explained with dark energy," Lamer said.

Since dark energy is contributing to the stretching of the universe, and speeding up the process of galaxies receding from each other, it hampers the growth of massive galaxy clusters in more recent times.

To test dark energy, scientists compare frequency of these massive clusters today with earlier times. If there were no dark energy, they would expect clusters to grow relatively quickly, so the largest clusters we see now would be very small at half the age of the universe, and there would be no gigantic clusters.

"The fact that we do find these clusters is a clear confirmation of dark energy," Lamer told SPACE.com. "This was a very long time ago and it's actually about as massive as the most massive ones we observe today. Without dark energy we would observe much more massive clusters and many more of these massive clusters than we actually do."

The discovery will be detailed in a forthcoming issue of the journal Astronomy & Astrophysics.

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