Every wavelength of the electromagnetic spectrum that is not actively being generated, is decaying. At minimum the expanding universe saps frequency. More dire

Every wavelength of the electromagnetic spectrum that is not actively being generated, is decaying. At minimum the expanding universe saps frequency. More directly,radiation (light,photons) is absorbed and re-emmited by matter but the re-emitted energy is always at a lower frequency. This absorbed & re-emitted longer wavelength radiation is what can become trapped.