~380,000 years after the Big BangReputable sourceWell documented
The Cosmic Microwave Background: First Light
On the timeline · around ~380,000 years after the Big Bang · The First Instants
What happened
For its first few hundred thousand years the universe was an opaque fog of particles. About 380,000 years after the Big Bang it had cooled enough for electrons and nuclei to join into the first atoms — the epoch of recombination — and light was suddenly free to travel. That first light, stretched by the expanding universe into microwaves, still bathes the sky today as the cosmic microwave background.
Why it matters
The cosmic microwave background is the oldest light we can see. Mapped in detail by NASA's WMAP, it pins the age of the universe at 13.8 billion years and reveals that ordinary atoms make up only about 5% of the cosmos, with dark matter and dark energy the rest.