  Semi-visible jets, with a significant contribution to the event's missing
transverse momentum, can arise in strongly interacting dark sectors. This
results in an event topology where one of the jets can be aligned with the
direction of the missing transverse momentum. The first search for semi-visible
jets produced via a $t$-channel mediator exchange is presented. The analysis
uses proton-proton collisions with an integrated luminosity of 139 fb$^{-1}$
and a centre-of-mass energy of 13 TeV, collected with the ATLAS detector during
the Run 2 of the LHC. No excess over Standard Model predictions is observed.
Assuming a coupling strength of unity between the mediator, a Standard Model
quark and a dark quark, mediator masses up to 2.7 TeV are excluded at the 95%
confidence level. Upper limits on the coupling strength are also derived.
