But they don't. The conclusion is that there must be some form of matter surrounding the galaxy, in a spheroidal halo, that doesn't show up in telescopes. The problem is that we just don't know what form it takes. Some suggest that it is composed of neutrinos and the neutrinos have finite rest masses. Let's survey the candidates one by one starting with the collapsed stars.
Collapsed stars are dead stars.
All stars are in equilibrium under the influence of the inward force form gravity and the outward force resulting from the gas pressure and radiation due the thermonuclear furnace at the center of the star. When the furnace goes out, the star collapses. In a small or medium-sized star, the collapse is slow, and the result is a white hot dwarf, only slightly larger than the earth. If the star is about four solar masses, it ends its life with a tremendous explosion, the supernova. A large fraction of the star is blown off into space, but the explosion compresses the core with a tremendous pressure and the result is tiny neutron star only about 10 or so kilometers across.