Question: why do astronomers think that there is a great amount of mass in the halo of the milky way

title: Dark matter halo
text: The visible disk of the Milky Way Galaxy is embedded in a much larger , roughly spherical halo of dark matter . The dark matter density drops off with distance from the galactic center . It is now believed that about 95 % of the Galaxy is composed of dark matter , a type of matter that does not seem to interact with the rest of the Galaxy 's matter and energy in any way except through gravity . The luminous matter makes up approximately 9 x 10 solar masses . The dark matter halo is likely to include around 6 x 10 to 3 x 10 solar masses of dark matter . 

Answer: answer: The visible disk of the Milky Way Galaxy is embedded in a much larger, roughly spherical halo of dark matter. It is now believed that about 95 % of the Galaxy is composed of dark matter, a type of matter that does not seem to interact with the rest of the Galaxy's matter and energy in any way except through gravity.