Final answer:
In our galaxy's interstellar medium, there is on average about one atom or molecule in each cubic centimeter of space, which is dramatically less dense than the air we breathe on Earth.
Step-by-step explanation:
The average number of atoms or molecules in each cubic centimeter of space between the stars in our galaxy, also known as the interstellar medium, is indeed quite sparse when compared to the air we breathe here on Earth. With about 10¹⁹ atoms in each cubic centimeter of air, one might consider that quite dense. However, in the vastness of space, specifically within our Milky Way, the density drops substantially, averaging out to about one atom or molecule per cubic centimeter. This illustrates just how empty the interstellar space truly is, with intergalactic space being even more sparse, requiring the examination of a full cubic meter to find just one atom on average.