The cosmic microwave background has an approximate temperature of 2.7 kelvins.
This faint microwave glow is leftover radiation from the early universe. It was released about 380,000 years after the Big Bang, when the expanding universe cooled enough for electrons and protons to form neutral hydrogen. Light could then travel freely through space.
Cosmic expansion has stretched that ancient light into the microwave part of the electromagnetic spectrum. Today, its temperature is measured at about 2.725 K, although the exact value depends on the measurement and convention used. The radiation is extremely uniform, with tiny temperature variations that reveal the seeds of later galaxies and galaxy clusters.
A common mix-up is to confuse the background’s temperature with the universe’s age or with the temperature of empty space near Earth. The microwave background is not ordinary heat from stars; it is ancient radiation filling space in every direction. NASA’s COBE satellite, followed by WMAP and Planck, measured its spectrum and fluctuations with increasing precision.