The night sky looks dark, but it's actually filled with a faint glow. Most of this light is born in invisible clouds of cold gas — the very stuff from which new stars emerge. This gas barely emits, making it hard to spot. But sometimes it gives itself away through radio waves emitted by carbon monoxide (CO) molecules.
Astronomers have for the first time captured this faint glow from the entire sky, not just from individual galaxies. They used data from radio telescopes and a clever trick: comparing how light from distant galaxies is slightly altered as it passes through gas clouds. This allowed them to "develop" the invisible gas spread across the cosmic web.
It turns out there is about twice as much molecular gas in the Universe as seen in bright galaxies. This gas is consumed quickly — within just a billion years, which is three times less than the age of the Universe. That means galaxies constantly receive fresh fuel from outside; otherwise, stars would have stopped forming long ago. The rate of star formation is linked to the amount of gas by a simple law that appears to hold throughout the Universe.
The carbon monoxide (CO) molecule is the primary "beacon" for finding cold molecular hydrogen. Hydrogen itself barely emits, but CO shines in the radio range, so astronomers look for it instead.