  Results of a search for physics beyond the Standard Model in events
containing an energetic photon and large missing transverse momentum with the
ATLAS detector at the Large Hadron Collider are reported. As the number of
events observed in data, corresponding to an integrated luminosity of 36.1
$\textrm fb^{-1}$ of proton-proton collisions at a centre-of-mass energy of 13
TeV, is in agreement with the Standard Model expectations, exclusion limits in
models where dark-matter candidates are pair-produced are determined. For
dark-matter production via an axial-vector or a vector mediator in the
s-channel, this search excludes mediator masses below 750-1200 GeV for
dark-matter candidate masses below 230-480 GeV at 95% confidence level,
depending on the couplings. In an effective theory of dark-matter production,
the limits restrict the value of the suppression scale $M_{*}$ to be above 790
GeV at 95% confidence level. A limit is also reported on the production of a
high-mass scalar resonance by processes beyond the Standard Model, in which the
resonance decays to $Z\gamma$ and the Z boson subsequently decays into
neutrinos.
