Interstellar Medium

Basic Information

Space is not entirely empty. The interstellar medium is the extremely thin gas that fills interstellar space. In the Milky Way as a whole, the average density is around a million atoms per cubic meter (1 atom per cubic centimeter), but it varies widely. It is much less dense in the Local Bubble near the solar system, for example. Most of the matter is hydrogen - either atomic hydrogen or hydrogen ions, though hydrogen molecules (H₂) are found in molecular clouds. There is also some helium and even smaller amounts of other elements (such as in dust grains).

Putting that in perspective:
A million atoms per cubic meter still sounds pretty dense, but it's not. Your head (that thing on your shoulders) has about 4,560,000,000,000,000,000,000,000 atoms in it.
1,000,000 in a space a meter on a side is practically nothing compared to that. They ain't kidding when they say space is a vacuum. See also Space Exposure.

Sources

Bibliography

1. Wikipedia on Interstellar Medium

2. Wikipedia on Local Bubble

3. [http://education.jlab.org/qa/mathatom_03.html Jefferson Lab on number of atoms in your head.

Game and Story Use

• Can a hydrogen-burning rocket supplement it's fuel with atoms it sucks up out of space?