Microwaves are not only found in ovens.
In 1964, scientists Arno Penzias and Robert Wilson recorded microwaves emanating from every direction in the universe.
Penzias and Wilson determined that their source was probably the entire universe. The waves came from everywhere.
The only source that can be considered everywhere at once is the universe, if the theory of the big bang holds true.
Microwaves would have initiated at the time of the big bang. Since the big bang created all existing matter, everything would radiate the microwaves.
The temperature of the big bang may have been over 500 billion degrees.
The microwaves would have cooled a certain amount over the succeeding 10 to 20 billion years. Penzias and Wilson found that their microwaves were just the right temperature to have come from the original blast.
Their discovery helped secure the big bang scenario as the prominent theory of creation.