  Observables sensitive to the anomalous production of events containing
hadronic jets and missing momentum in the plane transverse to the proton beams
at the Large Hadron Collider are presented. The observables are defined as a
ratio of cross sections, for events containing jets and large missing
transverse momentum to events containing jets and a pair of charged leptons
from the decay of a $Z/\gamma^\ast$ boson. This definition minimises
experimental and theoretical systematic uncertainties in the measurements. This
ratio is measured differentially with respect to a number of kinematic
properties of the hadronic system in two phase-space regions; one inclusive
single-jet region and one region sensitive to vector-boson-fusion topologies.
The data are found to be in agreement with the Standard Model predictions and
used to constrain a variety of theoretical models for dark-matter production,
including simplified models, effective field theory models, and invisible
decays of the Higgs boson. The measurements use 3.2 fb$^{-1}$ of proton--proton
collision data recorded by the ATLAS experiment at a centre-of-mass energy of
13 TeV and are fully corrected for detector effects, meaning that the data can
be used to constrain new-physics models beyond those shown in this paper.
