CERN-LHC. 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. Numerator and denominator ($\geq 1$ jet): - $p_\text{T}^\text{miss} > 200$ GeV - no additional electron or muon with $p_\text{T}$(lepton)>7 GeV and |$\eta$(lepton)|<2.5 - |$y$(jet)|<4.5 and $p_\text{T}$(jet)>25 GeV - $\Delta\phi$(jet,$p_\text{T}^\text{miss}$) > 0.4 for the four leading jets with $p_\text{T}$(jet)>30 GeV - leading $p_\text{T}$(jet)>120 GeV and |$\eta$(jet)|<2.4 Numerator and denominator (VBF): - $p_\text{T}^\text{miss} > 200$ GeV - no additional electron or muon with $p_\text{T}$(lepton)>7 GeV and |$\eta$(lepton)|<2.5 - |$y$(jet)|<4.5 and $p_\text{T}$(jet)>25 GeV - $\Delta\phi$(jet,$p_\text{T}^\text{miss}$) > 0.4 for the four leading jets with $p_\text{T}$(jet)>30 GeV - leading pT(jet)>80 GeV and subleading pT(jet)>50 GeV - $m_\text{jj}$>200 GeV - no additional jets with $p_\text{T}$(jet)>25 GeV inside rapidity interval Denominator only ($\geq 1$ jet and VBF): - leading $p_\text{T}$(lepton)>80 GeV and |$\eta$(lepton)|<2.5 - subleading $p_\text{T}$(lepton)>7 GeV and |$\eta$(lepton)|<2.5 - 66 GeV $< m_{\ell\ell}<$ 116 GeV - $\Delta R$(jet,lepton)>0.5, otherwise jet is removed