It is true that the Big Bang theory suggests that there was no light in the universe before the time known as the "recombination era." During this era, which occurred approximately 380,000 years after the Big Bang, the universe had cooled down enough for atoms to form, allowing photons (particles of light) to travel more freely.
The cosmic microwave background (CMB) radiation is one of the key pieces of evidence supporting the Big Bang theory. The CMB is a faint glow of radiation that permeates the universe, and it is essentially the "afterglow" of the Big Bang itself. It was emitted when the universe had cooled sufficiently for electrons to combine with atomic nuclei, forming neutral atoms. This event, called recombination, allowed photons to travel freely without being constantly scattered by charged particles.
The CMB radiation was initially in the form of extremely energetic photons, but as the universe expanded over billions of years, these photons were stretched to longer wavelengths, shifting into the microwave region of the electromagnetic spectrum. This is why it is called the cosmic microwave background.
Scientists have been able to detect and study the CMB using specialized instruments, such as the Wilkinson Microwave Anisotropy Probe (WMAP) and the Planck satellite. By observing the CMB, scientists can gain valuable insights into the early universe, its composition, and its evolution.
It's important to note that while we cannot directly observe events that occurred before the recombination era, the CMB radiation provides a window into the state of the universe at that time, giving us valuable information about the early stages of our universe's history.