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