  The central dark-matter fraction of galaxies is sensitive to feedback
processes during galaxy formation. Strong gravitational lensing has been
effective in the precise measurement of the dark-matter fraction inside massive
early-type galaxies. Here, we compare the projected dark-matter fraction of
early-type galaxies inferred from the SLACS strong-lens survey, with those
obtained from the EAGLE, Illustris, and IllustrisTNG hydro-dynamical
simulations. Previous comparisons with some simulations revealed a large
discrepancy, with considerably higher inferred dark-matter fractions -- by
factors 2-3 -- inside half of the effective radius in observed strong-lens
galaxies as compared to simulated galaxies. Here, we report good agreement
between EAGLE and SLACS for the dark-matter fractions inside both half of the
effective radius and the effective radius as a function of the galaxy's stellar
mass, effective radius, and total mass-density slope. However, for IllustrisTNG
and Illustris, the dark-matter fractions are lower than observed. This work
consistently assumes a Chabrier IMF, which suggests that a different IMF
(although not excluded) is not necessary to resolve this mismatch. The
differences in the stellar feedback model between EAGLE and Illustris and
IllustrisTNG, are likely the dominant cause of the difference in their
dark-matter fraction, and density slope.
