Deep space is so empty that it contains only about one hydrogen atom per cubic centimeter.

The Intergalactic Medium (IGM) consists of a rarefied plasma of hydrogen and helium that fills the vast distances between galaxies. While the density of the universe varies, the average density of the IGM is estimated by astrophysicists at the California Institute of Technology to be roughly one atom per cubic meter. However, in the slightly more crowded Interstellar Medium (ISM) within galaxies, the density averages about one atom per cubic centimeter.In contrast, the Earth's atmosphere at sea level contains approximately 2.5 x 10^19 molecules per cubic centimeter. This number is known as the Loschmidt constant. This means that terrestrial air is roughly 25 quintillion times denser than the matter found in the deep reaches of space. This extreme vacuum is far more empty than any vacuum ever created in a laboratory on Earth.Despite this incredible sparseness, the sheer scale of the universe means this matter adds up. Over millions of years, gravitational instabilities cause these lone atoms to clump together. These clumps eventually form giant molecular clouds, which are the birthplaces of stars and planetary systems. This process was first described in detail by Sir James Jeans in 1902 through the concept now known as the Jeans Instability.