At the moment of the Big Bang when the universe was first born, when the universe was about the size of an elementary particle. There is no light at the beginning of the universe, and it will not be visible until about 300,000 years after the birth of the universe.

In the universe after birth, atoms, electrons, and atomic nuclei are separated. Since the atoms were not solidified as matter, the electrons were flying around freely. When the universe expands and cools, the muddy universe becomes transparent, and the inside becomes empty And since it solidifies as a substance, there are no free electrons. Since the electrons that were floating around are gone, there are many gaps and the light can go straight. It took 300,000 years for the substance to solidify and become visible.