cosmic microwave background radiation

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Microwave radiation that penetrates space throughout the universe, postulated to be residual electromagnetic radiation originally emitted by warm matter in the universe approximately 400,000 years after the Big Bang.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (cosmology) the cooled remnant of the hot big bang that fills the entire universe and can be observed today with an average temperature of about 2.725 kelvin