The "cosmic dark ages" refers to a period during the early universe when sources of light were cloaked in a dense fog of neutral hydrogen gas. While light can now travel in all directions across the universe, rendering it transparent, the early universe was shrouded in hydrogen, which absorbed the light emitted by the first stars and sources of radiation.
For the first 380,000 years after the Big Bang, all matter and energy existed as an extremely hot, dense expanding ball of ionized plasma. At this point, collisions between subatomic particles in this high-energy soup prevented the particles from acquiring electrons to form stable atoms. Once the universe had sufficiently expanded and cooled, however, subatomic particles could acquire electrons to form neutral hydrogen atoms, and the cosmic dark ages began.
