Part 5/10:
The cosmic microwave background (CMB) acts as a relic of the early universe, providing essential clues about the presence of dark matter. Initially, the universe was a hot, dense plasma where electrons, protons, and neutrons floated freely, forming an opaque medium that blocked light. Approximately 380,000 years after the Big Bang, the universe cooled enough for hydrogen atoms to form, allowing photons to travel freely—this is the CMB we observe today.