  NASA's Cosmic Background Explorer (COBE) Satellite has recently made the most
accurate measurement of the temperature of the Universe determining it to be
$2.726\pm 0.01\,$K. In trying to understand why the temperature has this value,
one is led to discover the most fundamental features of the Universe---an
early, radiation-dominated epoch, enormous entropy per nucleon, synthesis of
the light elements around three minutes after the bang, and a small excess of
matter over antimatter---as well as some of the most pressing issues in
cosmology today, the development of structure in the Universe and the
identification of the nature of the ubiquitous dark matter.
