On average, the interstellar medium in our region of the Milky Way galaxy has a density of about one atom of gas per cubic centimeter. By contrast, Earth’s atmosphere at sea level contains about 10¹⁹ (or ten billion billion) gas molecules per cubic centimeter. There is also about one dust particle per 10 million cubic meters in the local interstellar medium.

In some places, the interstellar medium can be much denser. When there is a large enough concentration of gas and dust in a given place, the interstellar medium can form clouds that are thousands of times denser than the one atom per cubic centimeter. Even so, these interstellar clouds are far less dense than the best laboratory vacuum chambers can produce on Earth.