  We study the dark matter (DM) assembly in the central regions of massive
early-type galaxies up to $z\sim 0.65$. We use a sample of $\sim 3800$ massive
($\log M_{\rm \star}/M_{\rm \odot} > 11.2$) galaxies with photometry and
structural parameters from 156 sq. deg. of the Kilo Degree Survey, and
spectroscopic redshifts and velocity dispersions from SDSS. We obtain central
total-to-stellar mass ratios, $M_{\rm dyn}/M_{\rm \star}$, and DM fractions, by
determining dynamical masses, $M_{\rm dyn}$, from Jeans modelling of SDSS
aperture velocity dispersions and stellar masses, $M_{\rm \star}$, from KiDS
galaxy colours. We first show how the central DM fraction correlates with
structural parameters, mass and density proxies, and demonstrate that most of
the local correlations are still observed up to $z \sim 0.65$; at fixed $M_{\rm
\star}$, local galaxies have larger DM fraction, on average, than their
counterparts at larger redshift. We also interpret these trends with a non
universal Initial Mass Function (IMF), finding a strong evolution with
redshift, which contrast independent observations and is at odds with the
effect of galaxy mergers. For a fixed IMF, the galaxy assembly can be
explained, realistically, by mass and size accretion, which can be physically
achieved by a series of minor mergers. We reproduce both the $R_{\rm e}-M_{\rm
\star}$ and $M_{\rm dyn}/M_{\rm \star}-M_{\rm \star}$ evolution with stellar
and dark mass changing at a different rate. This result suggests that the main
progenitor galaxy is merging with less massive systems, characterized by a
smaller $M_{\rm dyn}/M_{\rm \star}$, consistently with results from halo
abundance matching.
