Ok so I'm a bit confused as to how to attempt to answer this question? Describing its significance vs explaining its significance? Here is my attempt, please be ruthless when correcting me as I feel like there are some gaps in my knowledge and misconceptions I have: Where I put '/' I wasn't sure which phrase or word was better to use or correct, please can you tell me which one? The 3K cosmic microwave background radiation is electromagnetic radiation in the microwave region of the electromagnetic spectrum and can be detected from all directions. It is the predominant electromagnetic radiation emitted from space. It correspond to the temperature at which space/ the universe is now at-3K. Finally it is uniform and isotropic/ its intensity is the same in all directions. The 3K CMB provides evidence for the hot Big Bang model of the universe. According to the hot Big Bang model of the universe; - the universe began from a singularity called the Big Bang from which space and time evolved -initially the universe was very hot and emitted electromagnetic radiation mainly of shorter wavelengths, this electromagnetic radiation was almost perfectly uniform/isotropic and was given out intensely/emitted in all directions - 3x10^5 years after the Big Bang the universe became transparent to electromagnetic radiation and it was able to move freely through the universe -after the Big Bang the universe began to expand and cool -as a result these shorter wavelengths of electromagnetic radiation were stretched out so that they were of a longer wavelength -the current temperature of universe is 3K The fact that the CMB is in the microwave region of the em spectrum supports the idea of the expansion of the universe , and that the universe must have originally begun from a single point- the Big Bang. Secondly the presence of the CMB corresponds to the temperature of the universe/space-3K, supporting the idea of the cooling of the universe and the fact that initially the universe must have been very hot. Finally the fact that the CMB is almost perfectly uniform/isotropic supports the idea that the universe was initially isotropic, however there were some irregularities in the density of the universe, which were the seedlings for the formation of galaxies and stars that were to come after the Big Bang.