At first, the formed matter and light seemed trapped. The latter collided with the first one, being incapable of travel freely as it does now. The early cosmos was so dense and hot that electrons could not remain attached to the nuclei of the atoms. However, as the Universe continued to expand and the temperature cooled down (3,000 °C approximately), the electrons started to remain next to their nuclei, preventing the light from scattering. This stage is known as Recombination or Surface of the Last Scattering since the atoms merge and, from that very moment (380,000 years after the Big Bang), light can travel freely in the Universe.
The light we detect and see today thanks to the microwave telescopes, it has traveled 13.7 billion years, and it has “cooling down” with the Universe, reaching a temperature of only 2.7 K (-271 °C).
Electrons are not bound to the atomic nuclei, and photons collide continuously with electrons and ions.
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