The interstellar medium (or ISM) is the gas and dust that pervade interstellar space and blends smoothly into the surrounding intergalactic space. For centuries, scientists believed that the space between the stars was empty. It was only in the last century that observations of interstellar material suggested that it was not even uniformly distributed through space, but that it had a unique structure. The interstellar medium consists of an extremely dilute mixture of ions, atoms, molecules, larger dust grains, cosmic rays, and magnetic fields. The matter consists of about 99% gas and 1% dust by mass. Densities range from a few thousand to a few hundred million particles per cubic meter with an average value in the Milky Way Galaxy of a million particles per cubic meter. The gas is roughly 89% hydrogen and 9% helium and 2% elements heavier than hydrogen or helium by number of nuclei.