cosmic dark ages
After the brilliant fireball of the Big Bang and the flash of light released at recombination, you might imagine the universe stayed lit. Instead the lights went out. For a long stretch — hundreds of millions of years — there were no stars, no galaxies, nothing burning anywhere. The afterglow of the Big Bang had faded to invisible infrared and the first stars had not yet switched on. This long, lightless intermission is the cosmic dark ages: the era between the release of the cosmic microwave background and the ignition of the first stars.
Recombination, around 380,000 years after the Big Bang, filled the universe with neutral hydrogen and helium and let the cosmic microwave background fly free. But that relic light kept stretching and cooling as space expanded, sliding out of the visible range, and there were no other sources of light. The universe was a dark, nearly uniform sea of cold neutral gas, gently lit only by the fading microwave glow. Quietly, though, gravity was at work: the faint primordial density seeds were pulling gas together, slowly building up the dense clumps that would eventually collapse into the first stars. The dark ages lasted from roughly 380,000 years to a few hundred million years after the Big Bang.
The dark ages are a frontier of observational astronomy precisely because there was so little light to see. We cannot photograph stars from this era because there were none. But the era is not entirely invisible: the cold neutral hydrogen filling it emits and absorbs a faint radio signal at a wavelength of 21 centimeters, and astronomers are building experiments to map that whisper, hoping to chart how gas clumped and when the first stars began to glow. The dark ages matter because they are the gestation period of cosmic structure — the patient, dark interval during which gravity assembled the first objects out of nearly featureless gas, ending the era of pure smoothness and beginning the era of stars.
It is a striking thought: for hundreds of millions of years the entire universe had no stars in it at all. If you could have floated through space then, you would have seen utter blackness in every direction, swimming through cold hydrogen gas at a few tens of degrees above absolute zero, with not a single point of starlight anywhere.
For a long stretch after recombination, the universe held no stars — only dark, cold gas.
The dark ages were not pitch black in every sense — the cosmic microwave background still bathed everything, and 21-cm radio emission was present. 'Dark' means no stars or galaxies had yet formed, not a total absence of any radiation.