The density of material that would be obtained if all the matter contained in galaxies were smoothed out across the Universe. Although stars and planets have densities greater than the density of water (about 1 g/cm3), the cosmological mean density is extremely low (less than 10−29 g /cm3, or 10−5 atoms/cm3) because the Universe consists mostly of virtually empty space between galaxies. The mean density of matter determines whether the Universe will continue to expand. See also Critical Density; Density Parameter.
Subjects: Astronomy and Astrophysics.