Vacuum is space devoid of matter. It theoretically contains one atom or ion every cubic metre. However, modern physics suggests that even this is not the complete picture, as atomic matter only makes up a few percent of the total mass of the Universe. The two other major contributors to the mass are dark matter and dark energy. Although the effects of these mysterious components can only be observed indirectly (through their influence on galactic and cosmological scales) it must be assumed that they too are present even in the remotest corners of the Universe. Dark energy in particular is thought to pervade all of space, and is actually described as a property (energy density) of the vacuum itself. This is the physical manifestation of the cosmological constant first introduced by Einstein.
Outer space has very low density and pressure, and is the closest physical approximation of a perfect vacuum. But no vacuum is truly perfect, not even in interstellar space, where there are still a few hydrogen atoms per cubic meter. All of the observable universe is filled with large numbers of photons, the so-called cosmic background radiation, and quite likely a correspondingly large number of neutrinos. The current temperature of this radiation is about 3 K, or −270 degrees Celsius or −454 degrees Fahrenheit.