Back at the dawn of the Universe, astronomers have found a stacks on of cosmic proportions. At least 21 galaxies, forming stars at a tremendous rate, are merging together in the early stages of the formation of a galaxy cluster. And it's all happening 13 billion light-years away - just 770 million years after the Big Bang itself.

This is the earliest protocluster discovered yet, named LAGER-z7OD1, and today it has probably evolved into a group of galaxies 3.7 quadrillion times the mass of the Sun.

Such a large protocluster, so early in the Universe - barely a cosmic eyeblink since the curtain was raised on life, the Universe and everything - could contain some vital clues as to how the primordial smoke cleared and the lights switched on, sending light streaming freely through space.

Our Universe is a massively interconnected place. Galaxies may seem relatively self-contained, but more than half of all galaxies are gravitationally bound together in clusters or groups, huge structures of hundreds to thousands of galaxies.

The beginnings of such clusters are not unknown in the early Universe. Protoclusters have been found nearly as far as LAGER-z7OD1, some even much bigger, suggesting that clusters could begin assembling much faster than previously thought possible.

But LAGER-z7OD1, according to a team of researchers led by astronomer Weida Hu of the University of Science and Technology of China, is special. It can reveal clues about one of the most mysterious stages in the history of the Universe: the Epoch of Reionisation.

"The total volume of the ionised bubbles generated by its member galaxies is found to be comparable to the volume of the protocluster itself, indicating that we are witnessing the merging of the individual bubbles and that the intergalactic medium within the protocluster is almost fully ionised," they wrote in their paper.

"LAGER-z7OD1 thus provides a unique natural laboratory to investigate the reionization process."

Space, you see, wasn't always the lovely, see-through place it is today. For the first 370 million years or so, it was filled with a hot murky fog of ionised gas. Light was unable to travel freely through this fog; it scattered off free electrons and that was that.

Once the Universe cooled down enough, protons and electrons started to recombine into neutral hydrogen atoms. This meant that light - not that there was much, yet - could finally travel through space.

As the first stars and galaxies began to form, their ultraviolet light reionised the neutral hydrogen ubiquitous throughout the Universe: first in localised bubbles around the ultraviolet sources, and then larger and larger areas as the ionised bubbles connected and overlapped, allowing the entire spectrum of electromagnetic radiation to stream freely.

By about 1 billion years after the Big Bang, the Universe was completely reionised. This means that it's more challenging to probe beyond this point (about 12.8 light-years away), but it also means that the reionisation process itself is tricky to understand.

Ideally, you need really bright objects whose ionising radiation could cut through the neutral hydrogen, and that's what Hu and his team were looking for with the Lyman Alpha Galaxies in the Epoch of Reionization survey. These are small, early-Universe galaxies forming stars at an insane rate, which means they can be detected at quite large distances, well inside the Epoch of Reionisation. This makes them useful probes of the period.

In their search, the researchers found LAGER-z7OD1, an overdense region of galaxies in a three-dimensional volume of space measuring 215 million by 98 million by 85 million light-years. This volume contained two distinct sub-protoclusters merging together into one larger one, with at least 21 galaxies, 16 of which have been confirmed.

The total volume of ionised space around the galaxies was slightly larger than the volume of LAGER-z7OD1.

"This demonstrates substantial overlaps between individual bubbles, indicating that the individual bubbles are in the act of merging into one or two giant bubbles," the researchers wrote.

So not only does the protocluster represent an excellent example of its kind, providing a new datapoint for studying how these structures form and emerge, as well as star formation in the early Universe, it offers a one-of-a-kind window into the formation and combination of ionised bubbles in the middle of the Epoch of Reionisation.

What insights will emerge are yet to be discovered, though. As the researchers note, that will be the work of future, more powerful telescopes that will better be able to observe the finer details of the reionisation process.

The team's research has been published in Nature Astronomy.