The cosmic microwave background radiation
In 1965, while studying the ground noise of a radio antenna, two technicians of the Bell Telephone Laboratories heard a weak radio signal coming from space, whose peculiarity was to own the same intensity in every direction.
This signal represents the remainder of the ancient radiation, produced during the first phases of the Universal life, then moved to low frequencies because of the Universe expansion and cooling. Its wavelength, in fact, is about 0.2 cm, while its characteristic temperature is 2,7 K.
The microwave background radiation represents the best observative evidence of the Big Bang cosmologic model.
In 1992, the american satellite COBE (Cosmic Background Explorer) made an other important discovery: the cosmic background radiation presents unhomogeneous temperatures, called anisotropy, in all the space directions. This discovery gave a further confirmation to the Big Bang Theory: the primordial matter was not distributed in a homogeneous way, but there were slight differences in the density of several regions.
The COBE successor, the satellite WMAP, discovered the total density of our Universe, proving that it is not simply explanable considering the total amount of all the atomic mass. It is rather necessary to consider an invisible matter that permeates the Universe: the dark matter.