Dark mass or dark matter in our galaxy was discovered from much velocities of stars at distances from the galactic center.

Dark mass or dark matter in our galaxy was discovered from much higher velocities of stars at long distances from the galactic center.

Explanation:

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 in our galaxy, the Milky Way, has a great amount of dark matter in  comparison with ordinary matter (known visible 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 at zones where is supose there is no matter.