The strongest evidence for the Big Bang theory of the origin of the universe is the thermal radiation emitted by that explosion. This radiation has cooled over the years to only 3 K and is now mostly microwaves. Why should 3 K thermal radiation be microwaves?

Based on the Big Bang Theory, the universe should have radiation of the remnant heat from the initially very hot universe called the Cosmic Microwave Background, CMB, which is presently at a temperature of 2.725° above absolute zero temperature (approximately 3 K)

The blackbody spectrum of colder objects have progressively longer wavelengths. At the temperature of 3 K the microwave radiation is the most intense radiation emitted due to the motion of the elementary particles of an object

Therefore, a 3 k thermal radiation should be microwave

Explanation: