Most of the mass of the Milky Way exists in the form of:

a.) starsb.) gasc.) dustd.) dark matter

Answer: dark matter

It is believed that the Milky Way has 90% dark matter and only 10% ordinary matter (known matter). Because, like gravity, dark matter can not be observed directly, however its existence is inferred through the movement of the stars and the cosmic dust within the galaxy.

It is important to note that dark matter composition is unknown and corresponds to 80% of the matter in the universe. It does not emit or interact with any type of electromagnetic radiation, but it interacts with the known matter through gravity.