  Large galaxies may contain an "atmosphere" of hot interstellar X-ray gas, and
the temperature and radial density profile of this gas can be used to measure
the total mass of the galaxy contained within a given radius r. We use this
technique for 102 early-type galaxies (ETGs) with stellar masses M_* > 10^10
M_Sun, to evaluate the mass fraction of dark matter (DM) within the fiducial
radius r = 5 r_e, denoted f_5 = f_{DM}(5r_e). On average, these systems have a
median f_5 = 0.8 - 0.9 with a typical galaxy-to-galaxy scatter +-0.15.
Comparisons with mass estimates made through the alternative techniques of
satellite dynamics (e.g. velocity distributions of globular clusters, planetary
nebulae, satellite dwarfs) as well as strong lensing show encouraging
consistency over the same range of stellar mass. We find that many of the disk
galaxies (S0/SA0/SB0) have a significantly higher mean $f_5$ than do the pure
ellipticals, by Delta f_5 = 0.1. We suggest that this higher level may be a
consequence of sparse stellar haloes and quieter histories with fewer major
episodes of feedback or mergers. Comparisons are made with the Magneticum
Pathfinder suite of simulations for both normal and centrally dominant
"Brightest Cluster" galaxies. Though the observed data exhibit somewhat larger
scatter at a given galaxy mass than do the simulations, the mean level of DM
mass fraction for all classes of galaxies is in good first-order agreement with
the simulations. Lastly, we find that the group galaxies with stellar masses
near M_* ~ 10^11 M_Sun have relatively more outliers at low $f_5$ than in other
mass ranges, possibly the result of especially effective AGN feedback in that
mass range leading to expansion of their dark matter halos.
