The Cosmic Dark Ages
A hundred million years of neutral hydrogen, expanding in complete darkness.
- When
- —
- Duration
- 150 million years
- Known to
- ± 20 million years
- Era
- Primordial
After the cosmic microwave background is released and before the first stars ignite, the universe contains no sources of light at all — only cooling neutral hydrogen, drifting and slowly collecting under gravity into the halos that will become the first galaxies. It is the one interval defined by the absence of events. Radio astronomy is beginning to probe it through the 21-centimetre line of neutral hydrogen, the only signal the period emits.