At the beginning there was light (photons!). The primordial Universe was hot, matter was completely ionized and its dynamics was governed by a huge radiation bath. As the Universe expanded and cooled, the atoms formed (380 000 years after the Big-Bang) and this radiation bath could finally escape carrying the imprints of the forming Large Scale Structure (the small temperature fluctuations are like a xerox copy of the matter fluctuations at the time). Nowadays, with almost 14 thousand million years of Cosmic History, this radiation bath forms the Cosmic Microwave Background Radiation (CMBR), detected for the first time in 1965 and mapped for the first time with the DMR instrument aboard COBE satellite in 1992. George Smoot, PI of COBE/DMR, the GEM mentor, was awarded the Physics Nobel Prize in 2006 for this discovery.