Astronomers have revealed the most distant cluster of galaxies ever observed, caught at a never-before-seen stage of development. Cosmos-Aztec3 has been described as a "metropolis in the making", because such clusters are believed to grow like cities, absorbing outlying villages. It lies 12.6 billion light years away, and appears to be just tens or hundreds of millions of years old.
Previously discovered galaxy clusters have been billions of years older. By contrast, the light from the "protocluster" Cosmos-Aztec3 left when the Universe itself was just one billion years old. Galaxy clusters grow over billions of years, drawing together many galaxies and huge amounts of gas to form the largest structures in our Universe.