how dense is the matter between the stars
Interstellar space is very cold. The density of interstellar matter is extremely low. It averages roughly 10 6 atoms per cubic meter--just 1 atom per cubic centimeter--but densities as great as 10 9 atoms/m 3 (1000 atoms/cm 3) and as small as 10 4 atoms/m 3 have been found.