Final answer:
The interstellar medium, by mass, consists mainly of gas and dust, comprising about 15% of the mass relative to stars, with gas making up 99% of that material.
Step-by-step explanation:
By mass, the interstellar medium in our region of the Milky Way consists primarily of gas and dust, which accounts for about 15% of the mass compared to the mass contained in stars. Of this interstellar medium, approximately 99% is gas with hydrogen and helium being the most abundant elements, and about 1% is made up of solid interstellar dust grains. Astronomers have estimated that the total mass of this gas and dust in the Milky Way Galaxy is around 10 billion times the mass of the Sun.