This period lasted from 370 thousand to 1 billion years after the Big Bang. During this time, matter in the universe was grouped under the influence of gravity. New stars were formed and merged into larger structures.
For the first three million years, the universe was filled with orange light emitted by photons released from hydrogen atoms. Over time, the wavelength shifted to infrared, and thus the wavelength range was invisible to the human eye, and the universe became a dark place again.
Only between 200 and 500 million years after the Big Bang did the first stars and galaxies form. So far, astronomers have not been able to observe a single star formed during this period. At the final stage of this period, the first clusters and superclusters of galaxies formed and the universe slowly filled with light and began to resemble the one we know today.
