  A search is performed for dark matter particles produced in association with
a resonantly produced pair of $b$-quarks with $30< m_{bb}<150$ GeV using 140
fb$^{-1}$ of proton-proton collisions at a center-of-mass energy of 13 TeV
recorded by the ATLAS detector at the LHC. This signature is expected in
extensions of the Standard Model predicting the production of dark matter
particles, in particular those containing a dark Higgs boson $s$ that decays
into $b\bar{b}$. The highly boosted $s \rightarrow b\bar{b}$ topology is
reconstructed using jet reclustering and a new identification algorithm. This
search places stringent constraints across regions of the dark Higgs model
parameter space that satisfy the observed relic density, excluding dark Higgs
bosons with masses between $30$ and $150$ GeV in benchmark scenarios with $Z'$
mediator masses up to $4.8$ TeV at 95% confidence level.
