  A search for dark-matter particles in events with large missing transverse
momentum and a Higgs boson candidate decaying into two photons is reported. The
search uses $139$ fb$^{-1}$ of proton-proton collision data collected at
$\sqrt{s}=13$ TeV with the ATLAS detector at the CERN LHC between 2015 and
2018. No significant excess of events over the Standard Model predictions is
observed. The results are interpreted by extracting limits on three simplified
models that include either vector or pseudoscalar mediators and predict a final
state with a pair of dark-matter candidates and a Higgs boson decaying into two
photons.
