Yi-Kuan Chiang

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.

380 thousand years after the Big Bang, the universe cooled enough for light to decouple from plasma. That cosmic microwave background still pervades space, like the first chord of a symphony, the echo of a fiery beginning.

The night sky looks black, but if you remove the stars and accumulate light for a long time, a faint glow emerges — the summed light of all galaxies born over 13.8 billion years. Astronomers call it the extragalactic background light.