The microwave background radiation is a result of the big bang.The big bang also resulted in a uniform distribution of background:

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The big bang created a high-energy plasma consisting of protons, neutrons, electrons, neutrinos, and photons. As the universe expanded and cooled down, the plasma started to form atoms and the photons were no longer absorbed and scattered. These photons form the microwave background radiation we see today. Neutrinos are also created in the high-energy plasma and like photons, have free stream since the universe was transparent to them. Therefore, the distribution of neutrinos is also uniform throughout the universe.

The Microwave Background Radiation Is a Result of the Big