Observing stars we can calculate their velocity, orbital recession, magnitude and distance using simple physics. The stars rotating in a given galaxy show two different types of motion depending on their distance from the center of the galaxy. The stars closer to the center of the galaxy exhibit the rigid body motion where all the stars have the same period of rotation. However the more distant stars show the differential rotation where each of them have their own separate period of rotation.
As can be seen in the graph above the rotational velocity ‘v’ of the stars do not fall off with the with the distance from the center of the system, but is rather slowly rising with the radius. According to the data we should have ten times more mass then what we can see to account for the observed velocities. Something must be there and that is what astrophysicists refer to as dark matter. These are the stellar objects that are too dim to be detected directly using current technology. These objects don’t emit, absorb or scatter light at all. Stellar remnants such as white dwarfs, neutron stars, black holes, Weakly Interacting Massive Particles (WIMPs) predicted by the Standard Model of Particle Physics and neutrinos qualify as dark matters.