Ghostly galaxies in the distant universe are almost certainly the culprits behind reionisation, a change in intergalactic gas that allows us to see across the cosmos. About 300,000 years after the big bang, the charged hydrogen that filled the universe became neutral and opaque, creating a cosmic fog that blotted out visible light for a billion years. Then some form of radiation broke up the hydrogen, reionising it and making it clear.
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