Cosmic Microwave Background Radiation

Cosmic Microwave Background Radiation is the radiation by the Universe in its early stages (here, here and here), shortly after the Photon Epoch, about 400,000 years after the Big Bang or 13.7 billion years ago. Since it was formed close to the Big Bang, it can be used as a "timer" for the age of the Universe (because of light aging).  Today it counts for roughly .00005 of the density of the Universe.  It keeps the average temperature of space around 2.3 Kelvin.  If the CMBR (short for Cosmic Microwave Background Radiation) ever dies out the Universe will end in a Big Freeze.  Microwave instruments detect this Radiation, and it is being studied for a look at the early Universe.