In a sky survey made in the near-infrared light spectrum, astronomers using Hubble found five tiny galaxies clustered together 13.1 billion light-years away.
That distance means Hubble has recorded images of them when very young, just 600 million years after the big bang, the space agency said in a release.
Such clusters, comprising hundreds to thousands of galaxies bound together by gravity, are the largest structures in the universe.
The developing cluster, or protocluster, observed as it looked 13 billion years ago, has likely grown into one of today's massive galactic collections, astronomers said.
"These galaxies formed during the earliest stages of galaxy assembly, when galaxies had just started to cluster together," Michele Trenti of the University of Colorado at Boulder said.
"The result confirms our theoretical understanding of the buildup of galaxy clusters. And, Hubble is just powerful enough to find the first examples of them at this distance."