400 thousand–400 million years after the Big Bang
After the emission of the Cosmic Microwave Background, or CMB, the Universe was made up primarily of neutral gas.
There were denser regions, with slightly more gravitational pull than the less dense regions. The extra gravitational pull meant that these regions accumulated more matter, and so got denser. As it got denser, it got slightly hotter, but not yet hot enough to start forming any stars.
Other than this, the Universe was a fairly dull place, with nothing particularly exciting going on. Neutral hydrogen only shines very faintly in radio waves. Future experiments will hope to map out the neutral hydrogen in the early Universe.