  There are some basic differences between the observed properties of galaxies
and clusters and the predictions from current hydrodynamical simulations. These
are particularly pronounced in the central regions of galaxies and clusters.
The popular NFW (Navarro, Frenk, and White) profile, for example, predicts a
density cusp at the center, a behavior that (unsurprisingly) has not been
observed. While it is not fully clear what are the reasons for this
discrepancy, it perhaps reflects (at least partly) insufficient spatial
resolution of the simulations. In this paper we explore a purely
phenomenological approach to determine dark matter density profiles that are
more consistent with observational results. Specifically, we deduce the gas
density distribution from measured X-ray brightness profiles, and substitute it
in the hydrostatic equilibrium equation in order to derive the form of dark
matter profiles. Given some basic theoretical requirements from a dark matter
profile, we then consider a number of simple profiles that have the desired
asymptotic form. We conclude that a dark matter density profile of the form
1/(1+r/r_a)^3 is most consistent with current observational results.
