Just a few minutes after the Big Bang, the universe was a hot dense soup of hydrogen nuclei (protons), helium nuclei, electrons, and electromagnetic radiation (light) in the form of photons. But the photons were essentially trapped in the burgeoning universe—they could travel only short distances before being absorbed and then re-emitted by some particle of matter.
That all changed approximately 380,000 years after the Big Bang. The universe had then cooled to a more modest 3,000 °K (nearly 6000°F), at which point electrons and protons could capture one another and form into atomic hydrogen. The photons previously linked to unbound electrons and protons no longer had anything to interact with and were free to travel unimpeded. The universe became transparent to light and gave birth to the cosmic background radiation.
Of course, the expansion of the universe affected those photons just as it did the galaxies that formed a few hundred million years later. The light stretched to longer wavelengths, and those background photons now had wavelengths in the microwave part of the spectrum that reflected an overall temperature of 2.73°K. When astronomers discovered the cosmic microwave background in the 1960s, it provided the first observational proof that the universe began with a Big Bang. Modern measurements of this background light, such as this all-sky map [seen above] from the European Space Agency’s Planck satellite, reveal tiny temperature fluctuations (shown as color variations) that correspond to different densities—the seeds of the cosmic structure we see today.