Data from the Far Infrared Absolute Spectrophotometer (FIRAS) instrument aboard the Cosmic Background Explorer (COBE) satellite is a measurement of the cosmic microwave background radiation (CMBR) spectrum. The spectrum is that of a blackbody emitter having a temperature of 2.725 ±0.002 K.
For more information, see: NASA GSFC - Cosmic Background Explorer (COBE)
Raw data from: http://lambda.gsfc.nasa.gov/data/cobe/firas/monopole_spec/firas_monopole_spec_v1.txt
Planck law for blackbody radiation: https://en.wikipedia.org/wiki/Planck's_law
A well-deserved nod to Randall Munroe, who invented this meme: XKCD No. 54