What substance is believed to dominate the mass inventory of a spiral galaxy's halo?

Dark Matter

The halo, which is the vast, diffuse, spherical region surrounding the visible disk and bulge, is thought to be overwhelmingly dominated by Dark Matter. This mysterious, non-baryonic substance does not interact with light—it neither emits, absorbs, nor reflects it—making it invisible to traditional observation methods. However, its gravitational influence is crucial; its immense mass provides the necessary gravitational pull required to explain the observed rotational speeds of the galaxy's outer disk material. Estimates suggest that the dark matter component within the halo likely accounts for 75% to 90% of the galaxy's total mass, far outweighing all the stars, gas, and dust combined.