  Several extensions of the Standard Model predict the production of dark
matter particles at the LHC. An uncharted signature of dark matter particles
produced in association with $VV=W^\pm W^\mp$ or $ZZ$ pairs from a decay of a
dark Higgs boson $s$ is searched for using 139 fb$^{-1}$ of $pp$ collisions
recorded by the ATLAS detector at a center-of-mass energy of 13 TeV. The $s\to
V(q\bar q)V(q\bar q)$ decays are reconstructed with a novel technique aimed at
resolving the dense topology from boosted $VV$ pairs using jets in the
calorimeter and tracking information. Dark Higgs scenarios with $m_s > 160$ GeV
are excluded.
