Why is the temperature of the universe 2.726 Kelvin?

Turner, M S · Science · 1993

basic_science · Level V

Where this comes from

Abstract

The Cosmic Background Explorer satellite has recently made the most accurate measurement of the temperature of the universe, determining it to be 2.726 +/- 0.01 kelvin. 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 3 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.