Final answer:
The universe is mostly made up of gas and dust, with 99% of the material between the stars being gas and interstellar dust. The mass of the gas and dust in the Milky Way Galaxy is estimated to be about 15% of the mass contained in stars.
Step-by-step explanation:
The universe is still mostly made up of gas and dust. About 99% of the material between the stars is in the form of gas, including hydrogen and helium. Additionally, there is interstellar dust consisting of solid-frozen particles. Astronomers estimate that the total mass of gas and dust in the Milky Way Galaxy is equal to about 15% of the mass contained in stars.