  This paper reports a summary of searches for a fermionic dark matter
candidate in the context of theoretical models characterised by a mediator
particle exchange in the $s$-channel. The data sample considered consists of
$pp$ collisions delivered by the Large Hadron Collider during its Run 2 at a
centre-of-mass energy of $\sqrt{s} = 13$ TeV and recorded by the ATLAS
detector, corresponding to up to 140 $fb^{-1}$. The interpretations of the
results are based on simplified models where the new mediator particles can be
spin-0, with scalar or pseudo-scalar couplings to fermions, or spin-1, with
vector or axial-vector couplings to fermions. Exclusion limits are obtained
from various searches characterised by final states with resonant production of
Standard Model particles, or production of Standard Model particles in
association with large missing transverse momentum.
