A halo model is presented which possesses a constant phase space density (Q) core followed by a radial CDM-like power law decrease in Q. The motivation for the core is the allowance for a possible primordial phase space density limit such as the Tremaine-Gunn upper bound. The space density profile derived from this model has a constant density core and falls off rapidly beyond. The new model is shown to improve the fits to the observations of LSB galaxy rotation curves, naturally provides a model which has been shown to result in a lengthened dynamical friction time scale for the Fornax dwarf spheroidal galaxy and predicts a flattening of the density profile within the Einstein radius of galaxy clusters. A constant gas entropy floor is predicted whose adiabatic constant provides a lower limit in accord with observed galaxy cluster values. While `observable-sized' cores are not seen in standard cold dark matter (CDM) simulations, phase space considerations suggest that they could appear in warm dark matter (WDM) cosmological simulations and in certain hierarchically consistent SuperWIMP scenarios.

14 pages, 3 figures, accepted for publication in ApJ