Final answer:
Dark matter exists in a halo around the Milky Way extending over 200,000 light-years from the center, detected through its gravitational effects.
Step-by-step explanation:
The additional dark matter mass in the Milky Way Galaxy is located in a structure referred to as the dark matter halo. This halo extends well beyond the luminous boundaries of the galaxy, reaching at least 200,000 light-years from the galactic center. Although dark matter remains invisible because it does not emit light, its presence is inferred from gravitational effects, such as influencing the orbits of star clusters and dwarf galaxies. Moreover, dark matter can also be studied through its gravitational lensing effects on light from distant objects. Despite extensive research, the exact composition of dark matter is still a mystery, contributing to its intrigue in astronomical studies.