According to the theory of the Hot Universe, right after the Big Bang the Universe was a very hot and very dense blend of different particles. This particular state has no analog in present day physical reality.

About 370 000 years after the Big Bang, an emission appeared in the Universe that we know observe as "relic radiation". At that point in time, the Universe was significantly hotter and denser than it is now. The average temperature of the hot plasma, which uniformly filled the whole Universe was about 3000 К and the maximum of its radiation was in the visible part of the electromagnetic spectrum. With the expansion, the average temperature of the matter was continuously decreasing and at present is about 1000 times lower. Thus, now we must such for traces from this relic emission in the microwave part of the spectrum (at radio wavelengths).

First predicted by Gamow in 1946, the relic radiation was discovered (by chance) 20 years later. The maximum of the radiation is at wavelength of about 2 mm. It is characterised by very high level of isotropy, i.e. it is observed as a very uniform microwave radio signal, coming with the same intensity from all directions. It is also called Cosmic microwave background radiation (CMB or CMBR) because its source, which surrounds us from all directions lies further away (and further back in time) than any other object in the Universe – at a distance of about 14 billion light years – and appears as a background to all other emission. The spectrum of the CMB is well described by Plank’s law as a blackbody of temperature of about 3 K. The astronomers who discovered the CMB received the Nobel prize for Physics in 1978. The CMB observationally confirms the theory of the Hot Universe and together with the discovery that the galaxies are moving away from each other, is one of the greatest scientific achievements of the 20 century.

The CMB radiation comes to us from all directions very uniformly – the accuracy is about several hundredths of a percent. But in this uniform radiation, there are some “wrinkles” that are difficult to detect. They appeared during the earlier years of the life of the Universe due to unavoidable fluctuations arising from the quantum nature of matter. These wrinkles were discovered with observations from space and their discoverers received the Nobel prize for Physics in 2006. These barely visible clumps are the “seeds” for the future structural units of the Universe which under their own gravitation got denser gradually and at some point in time “expelled” themselves from the overall cosmological expansion. In the end they became the galaxies and clusters of galaxies that we observe today. The presence of the “pre-galactic” heterogenities in the early Universe, however, has left its clear mark on the map or the cosmic microwave background.