What observations indicate that in the past the universe had not only high density but also temperature?

With the help of a radio telescope, microwave radiation was detected coming from all directions. This radiation has a maximum at a wavelength of 1 mm and corresponds to the radiation of a substance heated to 2.7K. In the past, in the early stages of the evolution of the Universe, the density and temperature of this radiation were significantly higher. So in the past, not only the density, but also the temperature of the radiation associated with it were very high.