why do astronomers think that there is a great amount of mass in the halo of the milky way
Dark matter halo
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 . 
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 .
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.
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 .