  A direct search for Higgs bosons produced via vector-boson fusion and
subsequently decaying into invisible particles is reported. The analysis uses
139 $\text{fb}^{-1}$ of $pp$ collision data at a centre-of-mass energy of
$\sqrt{s}$=13 $\text{TeV}$ recorded by the ATLAS detector at the LHC. The
observed numbers of events are found to be in agreement with the background
expectation from Standard Model processes. For a scalar Higgs boson with a mass
of 125 $\text{GeV}$ and a Standard Model production cross section, an observed
upper limit of $0.145$ is placed on the branching fraction of its decay into
invisible particles at 95% confidence level, with an expected limit of $0.103$.
These results are interpreted in the context of models where the 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. Invisible decays
of additional scalar bosons with masses from 50 $\text{GeV}$ to 2 $\text{TeV}$
are also studied, and the derived upper limits on the cross section times
branching fraction decrease with increasing mass from 1.0 $\text{pb}$ for a
scalar boson mass of 50 $\text{GeV}$ to 0.1 $\text{pb}$ at a mass of 2
$\text{TeV}$.
