On Christmas Day 2021, a rocket lifted off from Kourou, French Guiana, carrying a telescope that had nearly been killed by budget overruns, political fights, and 25 years of painful uncertainty. Congress came within a vote of cancelling it in 2011. Yet the James Webb Space Telescope survived — and it has been repaying that faith in the most spectacular way imaginable. Now, data from JWST's landmark JADES survey has delivered something that stops astronomers mid-sentence.

A team led by Zihao Wu of the Center for Astrophysics, Harvard & Smithsonian, has identified a cluster of galaxies in the early universe so tightly packed that their density is roughly four times the surrounding average. These galaxies existed within the first 500 million years after the Big Bang — a cosmic eyeblink. They are not just close together; they appear to be talking to each other, interacting gravitationally, and possibly doing something remarkable: carving out a bubble of ionized hydrogen in the primordial fog around them. That process has a name — reionization — and it is one of the great unsolved stories of cosmic history.

Before the first stars and galaxies switched on, the universe was a cold, dark, opaque place. Hydrogen atoms blocked light from travelling freely. Then something lit the fuse. Energetic radiation from early galaxies began stripping electrons from hydrogen atoms, making the universe transparent.