Whenever a diffuse gravitating system condenses into a stable and more compact object, energy must be removed from it. This is a consequence of the Virial theorem, which mandates that in a stable system of gravitating particles there must be a proportional balance between the magnitudes of their kinetic and potential gravitational energies. The former must be equal to half the latter. For example, as a stable, hot, compact proto-star forms from a cold, diffuse cloud of gas and dust, energy conservation ensures that gravitational potential energy is converted into an equal amount of other forms of energy. The condensing gas cloud heats up and radiates energy. In this process the virial theorem mandates that the internal kinetic energy added to the gas be only half the converted potential energy, if the proto-star is to form quasi-statically and not to oscillate. The balance of half the converted potential energy must be dissipated from the condensing star as radiant energy during the normal process of star formation. In short, the virial theorem tells the star to shine, as it were; shine out into interstellar or ultimately intergalactic space, where plenty of room for emitted photons has been cleared by earlier condensations. The relevance of the virial theorem to cosmology is the following. The real universe is lumpy. It is composed of a hierarchy of stable (on human time scales) compact astronomical structures, ranging from gas clouds, planets and stars through globular clusters and galaxies to clusters of galaxies. All these structures are thought to have formed by the gravitational condensation of more diffuse arrangements of matter. Ultimately, all the radiation emitted by condensing matter over the estimated 13.8-billion-year life of the universe has been derived gravitational potential energy. Its emission has been mandated by the virial theorem. This radiation cannot have escaped from the universe in the way starlight escapes from a galaxy; after all by definition the universe includes all that now exists. This sea of trapped radiation may have been red-shifted as the universe has evolved, but it cannot have vanished. My question is: where is this radiation? And does it contribute to the background radiation?