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By analyzing new observations from NASA’s James Webb Space Telescope, a team led by Simon Lilly of ETH Zürich in Switzerland found evidence that galaxies that existed 900 million years after the Big Bang ionized the gas around them, causing it to become transparent.

And I mean really dark — the gas between stars and galaxies was opaque, so no light could shine through. As anyone who's ever looked through a telescope knows, that's no longer the case, since we can see celestial objects all throughout the universe from our vantage point here on Earth. But what caused the change in opacity?

Using observations from NASA's James Webb Space Telescope, an international team of astronomers led by Simon Lilly of ETH Zürich in Switzerland has an answer. The team looked back in time at galaxies from the end of the Era of Reionization, a dramatic period in the universe's history in which gas was heated, cooled and then reionized (given an electrical charge once again).

Looking at those early galaxies, which existed just 900 million years after the Big Bang, the team saw that most of the gas in the universe was somewhere between opaque and transparent. But directly around the galaxies, everything was clear.

(Image credit: NASA, ESA, CSA, Simon Lilly (ETH Zürich), Daichi Kashino (Nagoya University), Jorryt Matthee (ETH Zürich), Christina Eilers (MIT), Rob Simcoe (MIT), Rongmon Bordoloi (NCSU), Ruari Mackenzie (ETH Zürich); Image Processing: Alyssa Pagan (STScI) Ruari Macken)