Astronomers have detected two fully developed galaxy clusters that appear as they existed when the universe was 1.5 billion years old—about one-tenth of its current age.

The find, which was made by international teams using telescopes in Chile’s Atacama Desert and was reported in Nature and The Astrophysical Journal, was a surprise for the researchers. “Current theoretical and computer models suggest that [galaxy clusters] as massive as these should have taken much longer to evolve,” at least 3 billion years after the beginning of the universe, the European Southern Observatory reported.

“How this assembly of galaxies got so big so fast is a mystery. It wasn’t built up gradually over billions of years, as astronomers might expect,” Tim Miller, who was a lead author of one of the papers, stated. “This discovery provides a great opportunity to study how massive galaxies came together to build enormous galaxy clusters.”