The right answer is leftover radiation from the big bang.
Cosmic Microwave Background (CMB) is the name given to a very homogeneous electromagnetic radiation observed in all directions of the sky and whose peak emission is located in the field of microwaves (microwaves). . Anticipated in 1948 and discovered by chance in 1964, the CMB allows the scientific community to separate the various cosmological models, in particular by abandoning models based on the perfect cosmological principle and by prioritizing models based on the idea of Big Bang, which predict the emission of such thermal radiation at the time of the primordial Universe.
According to the standard model of cosmology, this fossil radiation was emitted approximately 380,000 years after the Big Bang, when the observable Universe was at the same time much smaller, dense and hot. It was then diluted and cooled by the expansion of the Universe and now has a very low average temperature, of the order of 3 Kelvin (K). The CMB is a very studied physical phenomenon since its discovery for two reasons: it corresponds to the oldest electromagnetic image that it is possible to obtain from the Universe and it presents tiny variations of temperature and intensity according to the observed direction, detailed anisotropies since the beginning of the 1990s which allow gathering a lot of information on the structure, the age and the evolution of the Universe.
