Scientists found out that in the Dark Age, when the universe was impermable to light, the hydrogen must still have absorbed its resonant waveleght and therefore it should be possible to find out how cosmic matter was distributed in the early universe. Read more:
[astro-ph/0312134] Measuring the Small-Scale Power Spectrum of Cosmic Density Fluctuations Through 21 cm Tomography Prior to the Epoch of Structure Formation
The HI 21 cm Line
Cosmic Microwave Background
What is the Inflation Theory