Cosmic Microwave Background

Examining the cosmic microwave background tells us much about the origins of the universe and supports the Big Bang theory. The cosmic microwave background essentially allows astronomers to see back in time to when the universe was only 380,000 years old and lets them see light from the most distant areas of the universe we observe.
The cosmic microwave background was first discovered by mistake when Penzias and Wilson became annoyed with the sounds being picked up by their satellite microwave antenna. Simultaneously, at Princeton, astronomers were trying to discern assumed characteristics of radiation left over from the Big Bang. This seemingly annoying sound turned out to be indicative of the cosmic microwave background.
The cosmic microwave background is comprised of photons we are observing from the end of the era of nuclei. This came from the heat from when the universe was created. Temperature fluctuations can indicate a great deal about the history of the universe. Astronomers believe that the fluctuations in the cosmic microwave temperature indicate much on the "fluctuations in the density of matter in the early universe." This can reveal information about the origins of galaxies and the large scale structure of the universe.  Cosmic Background Explorer (COBE) findings also indicate that the cosmic microwave background is not wholly uniform. These areas of fluctuations, and therefore increased density, explain the formation of galaxies. These new measuring technologies helped to restore faith in the Big Bang theory.

http://www.pbs.org/wgbh/aso/databank/entries/dp65co.html
http://wmap.gsfc.nasa.gov/universe/bb_cosmo_fluct.html