Astronomers have spotted one of the rarest and most extreme galaxy clusters in the universe and, behind it, an object that shouldn’t exist.

Galaxy clusters are collections of galaxies that orbit one another and are the most massive objects in the universe. The newly discovered cluster, first detected by the Hubble space telescope, is over 500 trillion times the mass of the sun. It is located approximately 10 billion light-years away. Because looking out into the distant cosmos means also looking back in time, the cluster formed during an era when the universe was a quarter its present age.

The cluster, named IDCS J1426.5+3508, is extreme because during this period in cosmic history, massive collections of galaxies were just beginning to form. Only one other cluster of comparable size has been seen at this distance and it is a lightweight compared to IDCS J1426.5+3508.

Adding to the object’s strangeness, a mysterious arc of blue light was seen just behind the galaxy cluster. Astronomers think this indicates another massive star-forming galaxy located even further away at an even earlier epoch.

Light from this more distant – and yet unnamed – galaxy has been highly distorted by an effect known as gravitational lensing. The gargantuan mass of the galaxy cluster bends and twists light coming from the distant galaxy, creating the strange blue arc.

The farther galaxy is estimated to be 10 to 13 billion light-years away and have a mass approximately 70 trillion times the sun.

The astronomers who spotted the blue arc calculated that the odds of finding such a massive galaxy so distant in the universe are practically nil: “For the observed magnitudes we expect to find no arcs over the entire sky as bright,” the team writes in one of three papers outlining their findings, published online in The Astrophysical Journal June 26.

Understanding how these objects came to be could help astronomers determine the history of galactic evolution throughout the history of the universe. The team hopes that the eROSITA mission – an x-ray telescope scheduled to launch next year and study galaxy clusters and dark matter – will provide more information on this bizarre finding.

Image: NASA/ESA/University of Florida, Gainsville/University of Missouri-Kansas City/UC Davis