Before the Universe cooled enough for atoms to exist, about 300,000 years after the big bang, the Universe was fog-like or opaque because photons could never move far in any direction without being absorbed.

Question

Quizplus · Accepted Answer

This is a correct description of the state of the Universe during the epoch known as recombination, which occurred about 300,000 years after the big bang. At that time, the temperature dropped enough for electrons and protons to combine and form neutral atoms, allowing photons to travel freely without being absorbed. Before that, the Universe was filled with a hot and dense plasma that scattered photons in all directions, making it opaque.

Before the Universe Cooled Enough for Atoms to Exist, About