The density of a NFW dark matter halo is shown color coded, along with its circular rotation curve. This results from fits to n-body codes that follow the cosmological evolution of dark matter, and its hierarchical merging to form dark matter halos. Perhaps surprisingly, no matter what the size of the halo, roughly the same universal density law arises, from dwarf galaxy mass, through galaxy mass, to cluster mass.
Although the rotation curves ultimately decline, they appear flat over a large region which is sampled by the HI disks.