Why do we believe that most of the mass of the milky way is in the form of dark matter? a. although dark matter emits no visible light, we have detected its radio emissions. b. the orbital speeds of stars far from the galactic center are surprisingly high. c. theoretical models of galaxy formation suggest that a galaxy cannot form unless it has at least 10 times as much matter as we see in the milky way disk. d. our view of distant galaxies is often obscured by dark blotches, which are presumably made of dark matter.

The orbital speeds of stars far from the galactic center are surprisingly high.

Dark matter is a type of matter, whose composition is unknown and which corresponds to 80% of the matter in the universe. Its name refers to the fact it does not emit or interact with any type of electromagnetic radiation, being completely transparent throughout the electromagnetic spectrum.  

In this sense, 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 far from the center and the cosmic dust within the galaxy.