In 1964, Arno Penzias and Robert Wilson of Bell Labs made the most important discovery of modern cosmology: the Cosmic Microwave Background. At the same time Jim Peebles from Princeton made a theoretical prediction that the universe must be filled with microwave radiation.
The existence of this radiation tells us that in the very beginning the universe was not only very dense, but also very hot, and most of the matter in the universe was in the form of hot gas.
This gas emitted radiation, which redshifted into microwave range as the universe expanded.
However, the story of the Cosmic Microwave Background (CMB) began earlier...
In 1940s George Gamow, Ralph Alpher, and Robert Herman studied the creation of chemical elements (process of nucleosynthesis) in the early universe. They found out that the early universe has to be hot, otherwise all the matter in the universe would form helium gas.
They also predicted the Cosmic Microwave Background radiation should have
temperature today of about 5 degrees Kelvin (the true number is 2.73). This
work was largerly forgotten.
Later, in 1953, Apher, Herman, and Follin greatly improved the predictions
of nucleosynthesis in the hot universe (now usually called
the Big Bang nucleosynthesis). They found that the universe should contain
about 25% of helium and 75% of hydrogen. To their surprise they
found that no heavy elements were formed at all.
Now we know that the heavy elements are made in stars. That was finally
understood in 1957. However, stars are not able to convert 25% of the
gas in the universe into helium. Thus, both the stars and the Big Bang
needed to create the heavy elements around us: Big Bang creates helium
(and trace amount of a few other isotopes) and stars create almost
everything else (cosmic rays create a few isotopes either).
The discovery of the CMB was the most important development leading to the
general acceptance of the Big Bang theory in mid 60s.
The CMB offers the best proof that the universe is highly (one part in
100,000) homogeneous on large scales.
The study of the CMB was significantly advanced very recently, with the
launch of
COBE satellite (COsmic Background Explorer) in 1989.
It worked for 4 years and was switched off in 1993.
COBE studied several different kinds of cosmic background, but with
respect to the CMB its mission was two-fold:
Origin of elements
COBE
COBE performed both these missions brilliantly.