  A search for dark matter particles is performed using events with a Z boson
candidate and large missing transverse momentum. The analysis is based on
proton-proton collision data at a center-of-mass energy of 13 TeV, collected by
the CMS experiment at the LHC in 2016-2018, corresponding to an integrated
luminosity of 137 fb$^{-1}$. The search uses the decay channels Z $\to$ ee and
Z $\to$ $\mu\mu$. No significant excess of events is observed over the
background expected from the standard model. Limits are set on dark matter
particle production in the context of simplified models with vector,
axial-vector, scalar, and pseudoscalar mediators, as well as on a
two-Higgs-doublet model with an additional pseudoscalar mediator. In addition,
limits are provided for spin-dependent and spin-independent scattering cross
sections and are compared to those from direct-detection experiments. The
results are also interpreted in the context of models of invisible Higgs boson
decays, unparticles, and large extra dimensions.
