A direct search for Higgs bosons that are produced via vector boson fusion and subsequently decay into invisible particles is reported.The analysis uses 139 fb$^{-1}$ of $pp$ collision data at $\sqrt{s}$ = 13 TeV recorded by the ATLAS detector at the LHC. The observed number of events are found to be in agreement with the background expectation from Standard Model processes. Assuming a 125 GeV scalar Higgs boson with a Standard Model production cross section, both the observed and expected upper limits on the branching fraction of its decay into invisible particles are derived to be 0.13 at 95$\%$ confidence level. These results are also interpreted in the context of models where the 125 GeV Higgs boson acts as a portal to dark matter, and limits are set on the scattering cross-section of weakly interacting massive particles and nucleons. The invisible decays of additional scalar bosons with masses from 50 GeV up to 1 TeV are also studied, and upper limits on the cross section times branching fraction are evaluated to be 0.97 pb for a scalar boson mass of 50 GeV, which falls with increasing mass to become 0.12 pb at a mass of 1 TeV.

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