Although the space between stars in our galaxy appears to be completely empty of matter, there in fact exists a very dilute gas

Although the space between stars in our galaxy appears to be completely empty of matter, there exists a very dilute gas even in “empty” space. This interstellar (meaning “between the stars”) medium varies in density (the amount of matter within a certain volume). Typical densities in the interstellar medium are one particle per cubic centimeter. Since hydrogen is the most single common element in the universe, chances are, if you were to select a box of space one centimeter wide on a side, there would only be one hydrogen atom inside that box. There are some regions known as molecular clouds where densities are 10,000 times greater than average. These are dark clouds of gas where it is cold enough for hydrogen to exist mostly in the form of molecules, and where young stars or protostars can form. In a cubic centimeter box here, you'd find 10,000 hydrogen molecules (H2). Although this sounds like a lot of material, a cloud like this resembles the best vacuums that we can generate here on Earth. In contrast, the air that you are breathing contains 10 molecules per cubic centimeter—that number is 1 followed by 19 zeroes!