Why does the Milky Way Galaxy contain dark matter?

The Milky Way contains dark matter because the observed orbital velocities of stars and gas in the outer parts of the galaxy remain constant rather than decreasing as expected. According to the laws of gravity, if the galaxy consisted only of visible matter, these velocities should drop off at greater distances from the center.12

The fact that these velocities remain high indicates that there is a significant amount of undetected, non-luminous material exerting a gravitational pull, which keeps the outer regions of the galaxy moving at high speeds. This material must extend well beyond the visible edges of the galaxy. While the exact nature of this dark matter remains a major question in astronomy, it is likely composed of undiscovered, nonbaryonic particles, such as weakly interacting massive particles (WIMPs), which do not emit or reflect light.12