The cosmic dark ages: a universe with no stars at all
What happened
After recombination's burst of light faded, the universe went dark again, and stayed that way for roughly the next 200 million years. There were no stars yet, and no galaxies to hold them, only a thinning sea of neutral hydrogen and helium gas drifting through space under gravity's slow pull. NASA's account of this stretch, called the cosmic dark ages, describes gravity gradually pulling the densest knots of that gas closer together, patiently building the raw material that would eventually collapse into the first stars. It is, by definition, the one era in cosmic history with no light of its own to observe directly.
Why it matters
The dark ages are where the universe's large-scale structure, the loose cosmic web that galaxies would later trace, first began to take shape under gravity alone, entirely in the dark. Every galaxy, star, and planet's location today still echoes the gas clumps that formed during this quiet, lightless stretch.
How we know
Because no light source existed during the dark ages themselves, this period is not observed directly, it is inferred from what bookends it: the smooth young universe the cosmic microwave background reveals at one end, and the fully reionized, star-filled universe astronomers observe from about a billion years onward at the other. The gap between those two snapshots is filled in by computer simulations of gravity acting on gas, built from the same physics used everywhere else in cosmology.
Sources
- NASA Science. Overview (Universe) · Reputable sourcescience.nasa.gov · The domain "science.nasa.gov" is on our Reputable source registry. · Link is live and its text matches the event's key terms (Jul 2026)
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Part of a timelineThe Early Universe7 events · The first billion years, from the fusion furnace of the Big Bang's opening minutes to the collision that reshaped the Milky Way, told through the instruments and satellites that found the evidence.View all →