Final answer:
The era referred to is 'the dark ages,' a period after the Big Bang from about 500,000 to 1 billion years when atoms first formed, the universe was transparent to light, and galaxies began to develop.
Step-by-step explanation:
The era of the universe lasting from about 500,000 years to about 1 billion years after the Big Bang, during which it was cool enough for neutral atoms to form, is known as 'the dark ages' of the cosmic timeline. Approximately 380,000 years after the Big Bang, the universe had cooled to a temperature of about 3000 K, leading to electrons combining with protons to form hydrogen atoms. This significant event made the universe transparent to light and allowed the Cosmic Microwave Background (CMB) radiation to travel freely. Without stars or galaxies to light up the celestial expanse, this era was literally dark. However, during this time, small fluctuations in the density of dark matter grew, creating gravitational pulls that later resulted in the formation of the earliest galaxies, approximately 400 to 500 million years post-Big Bang.
The transition from 'the dark ages' to an illuminated universe happened as the first stars and galaxies began to form and emit light, approximately 500 million years after the Big Bang. By the time the universe was about a billion years old, it was filled with radiation from these newly formed stars and galaxies. This marked the beginning of a new era, with galaxies merging to form larger structures, and clusters and superclusters of galaxies started to shape the universe into something more familiar to what we observe today.