  We contrast the gas kinematics and dark matter contents of $z=2$ star-forming
galaxies (SFGs) from state-of-the-art cosmological simulations within the
$\Lambda$CDM framework to observations. To this end, we create realistic mock
observations of massive SFGs ($M_*>4\times10^{10} M_{\odot}$, SFR
$>50~M_{\odot}$ yr$^{-1}$) from the TNG50 simulation of the IllustrisTNG suite,
resembling near-infrared, adaptive-optics assisted integral-field observations
from the ground. Using observational line fitting and modeling techniques, we
analyse in detail the kinematics of seven TNG50 galaxies from five different
projections per galaxy, and compare them to observations of twelve massive SFGs
by Genzel et al. (2020). The simulated galaxies show clear signs of disc
rotation but mostly exhibit more asymmetric rotation curves, partly due to
large intrinsic radial and vertical velocity components. At identical
inclination angle, their one-dimensional velocity profiles can vary along
different lines of sight by up to $\Delta v=200$ km s$^{-1}$. From dynamical
modelling we infer rotation speeds and velocity dispersions that are broadly
consistent with observational results. We find low central dark matter
fractions compatible with observations ($f_{\rm DM}^v(<R_e)=v_{\rm
DM}^2(R_e)/v_{\rm circ}^2(R_e)\sim0.32\pm0.10$), however for disc effective
radii $R_e$ that are mostly too small: at fixed $R_e$ the TNG50 dark matter
fractions are too high by a factor of $\sim2$. We speculate that the
differences in gas kinematics and dark matter content compared to the
observations may be due to physical processes that are not resolved in
sufficient detail with the numerical resolution available in current
cosmological simulations.
