What crucial component is the Milky Way's Halo primarily composed of, besides very old stars?

Dark matter

The Milky Way is modeled to possess a Halo, which is a much larger, fainter, and roughly spherical region that surrounds both the central bulge and the main stellar disk. While this halo does contain populations of very old stars, its most gravitationally significant and defining component is dark matter. Although dark matter cannot be observed directly because it does not emit or interact with electromagnetic radiation in a detectable way, its presence is inferred. The gravitational influence exerted by this mass is essential for correctly explaining the observed rotation speeds of visible matter, especially in the outer regions of the galaxy, which would otherwise rotate much slower than they are measured to.