In the first moments after the Big Bang, the universe was nearly uniform, like smooth dough. But within a billion years, in some places the dough began to rise faster: clumps of hundreds of galaxies emerged, which astronomers call protoclusters. These are the seeds of future giant clusters, the most massive bound structures in the universe.

The Chandra and JWST telescopes have for the first time caught such a protocluster in X-rays. The system JADES-ID1 is at a distance that light has traveled for 13.1 billion years, so we see it as it was one billion years after the Big Bang. The X-ray glow reveals hot gas heated to tens of millions of degrees. This gas, like steam over a boiling pot, already fills the space between galaxies, although according to calculations it should have remained cold.

The discovery raises the question: why did such a massive object arise so early? In standard cosmology, structures grow gradually, like a snowball rolling down a mountain. But JADES-ID1 is a snowball that is already at the foot of the mountain, even though the mountain has just begun. This means that in some regions of the early universe, growth was faster than models predict. This forces us to rethink our understanding of how the large-scale structure assembled.

Such early heating of gas could change the picture of cluster evolution. Perhaps massive structures were born in special regions where the density of matter was higher than average. Now astronomers will search for other similar objects to understand how often this happened.