The Chandra X-ray Observatory has caught enormous hot gas clouds in space in the act of merging to form a single massive galaxy cluster.
The clouds, which are many millions of degrees Celsius in temperature, each contain hundreds of galaxies.
The gas complex, known as Abell 2125, is about three billion light-years from Earth and is seen at a time about 11 billion years after the Big Bang.
Chandra is able to resolve individual galaxies within the gas clouds.
Galaxies are often found in groups or larger accumulations called clusters.
Our own galaxy, the Milky Way, belongs to the so-called Local Group, along with the larger spiral galaxy Andromeda (M31) and several smaller satellites, including the Large and Small Magellenic Clouds.
The build-up of these massive galaxy clusters is a step-by-step process that takes billions of years.
Exactly how long it takes for a cluster to form depends on several factors, including the density of subclusters nearby, the rate of expansion of the Universe and the relative amounts of dark energy and dark matter.
Astronomers think the very low concentration of iron atoms in one of the gas clouds in Abell 2125, suggests it is in the very early stages of cluster evolution.
The iron atoms produced by supernovas in the embedded galaxies must still be contained in and around those galaxies, perhaps in grains of dust that have not been well mixed with the gas.
Over time, as this anaemic cluster merges with other clusters and the hot gas pressure increases, the dust grains will be driven from the galaxies, mixed with the hot gas and destroyed, liberating the iron atoms.
"We may be seeing hot intergalactic gas in a relatively pristine state before it has been polluted by gas from galaxies," said Q. Daniel Wang of the University of Massachusetts in Amhurst, US.
The results will appear in a forthcoming issue of the Astrophysical Journal.