Space itself doesn't have a temperature, but the various fields and particles that inhabit it DO. The distances between particles in the outer Earth's atmosphere may seem enormous, and also those between atoms in interstellar space, but at the scale of the solar system or the Milky Way, even these rarified gases seem very dense and have a well-definable temperature. The outer 'thermosphere' of the Earth has a temperature of 500 K or so. The interstellar medium, with one atom per cubic centimeter has a temperature of 20 K, or a few thousand K depending on the environment. The definition of temperature under these conditions is perfectly well defined, even though the scales are vast.