  Several extensions of the Standard Model predict associated production of
dark-matter particles with a Higgs boson. Such processes are searched for in
final states with missing transverse momentum and a Higgs boson decaying to a
$b\bar b$ pair with the ATLAS detector using 36.1 fb$^{-1}$ of $pp$ collisions
at a center-of-mass energy of 13 TeV at the LHC. The observed data are in
agreement with the Standard Model predictions and limits are placed on the
associated production of dark-matter particles and a Higgs boson.
