The cosmic dark ages are a period of time in the early universe, starting around 380,000 years after the Big Bang and lasting until the first stars and galaxies formed, around 150 million years later. During this time, the universe was filled with neutral hydrogen gas that absorbed most of the light in the universe, making it opaque. Because of this, the universe was largely dark, with no visible light sources.
However, despite the darkness, the cosmic dark ages were an important period in the evolution of the universe. During this time, the first structures in the universe began to form, as gravity pulled matter together to create clumps of gas and dust. These clumps eventually formed the first stars and galaxies, which ionized the neutral hydrogen gas and ended the cosmic dark ages.
Overall, while the cosmic dark ages may have been a relatively "dark" period in the history of the universe, they were a crucial time in the formation and evolution of the universe as we know it today.
I believe you are referring to the concept of the "Dark Age" of the universe, which generally refers to a period in the early universe where there were no stars, galaxies or other luminous sources.
This period occurred after the cosmic microwave background radiation was emitted, which is thought to have happened around 380,000 years after the Big Bang. At this point, the universe became transparent to light, and it entered a period of relative darkness that lasted for several hundred million years.
During this time, the universe was filled with a hot, ionized gas, which made it opaque to light. As the universe expanded and cooled, this gas gradually became neutral, allowing light to pass through. Eventually, the first stars and galaxies began to form, marking the end of the Dark Age and the beginning of the Cosmic Dawn.
The study of the Dark Age is an important area of research in cosmology, as it can help us to understand how the first structures in the universe formed, and shed light on some of the fundamental questions about the nature of the universe itself