Astronomers used the afterglow of the big bang, hungry supermassive black holes, and space telescopes to find a rare ‘relaxed’ galaxy cluster.

The early universe was a stressful place for galaxies. Globs of tens to hundreds of neighboring galaxies, called galaxy clusters, would share a communal pool of hot gas—but not without drama. There was always another wayward galaxy crashing into the cluster, merging with one of the former occupants, and generally perturbing the gas pool, known as the intracluster medium.

That’s what makes the newly discovered galaxy cluster SPT2215 so special. Found about 8.4 billion-light years from Earth, astronomers recently captured views of SPT2215 as it existed when the universe was just 5 billion years old. On further study, they’ve deemed it one of the few “relaxed” galaxy clusters found from that period in the cosmos. It could lead scientists to revise how their models of how fast galaxies formed at the dawn of the universe.

“If the galaxy cluster is in the process of forming, we call it ‘disturbed’—it’s just kind of a mess,” says Michael Calzadilla, a PhD candidate in astrophysics at MIT and lead author of an April 19 paper in The Astrophysical Journal characterizing the newly discovered SPT2215 cluster with the help of multiple telescopes and flying observatories.

“If the gas is very round, very symmetrical, and looks kind of like a ball, it tells you that there haven’t been any recent interactions,” he says. “It’s very ‘relaxed.’” In other words, there are no galaxy mergers disrupting things, which seems to be the case with SPT2215.

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