How cold is space?

Space is almost empty, so temperature there is measured by the faint radiation that fills the universe, the afterglow of the Big Bang known as the cosmic microwave background. It gives a baseline of about 2.725 kelvin, just above absolute zero at −273.15 °C. Near a star, however, objects can get extremely hot: in direct sunlight near Earth, a spacecraft surface can exceed 120 °C while shaded parts drop below −100 °C. Because there is so little matter, heat moves mainly by radiation, not by conduction or convection. The coldest known natural place is the Boomerang Nebula, at about 1 kelvin.