  Ultra-diffuse galaxies (UDGs) are the lowest-surface brightness galaxies
known, with typical stellar masses of dwarf galaxies but sizes similar to
larger galaxies like the Milky Way. The reason for their extended sizes is
debated, with suggested internal processes like angular momentum, feedback or
mergers versus external mechanisms or a combination of both. Observationally,
we know that UDGs are red and quiescent in groups and clusters while their
counterparts in the field are blue and star-forming. This dichotomy suggests
environmental effects as main culprit. However, this scenario is challenged by
recent observations of isolated quiescent UDGs in the field. Here we use
$\Lambda$CDM cosmological hydrodynamical simulation to show that isolated
quenched UDGs are formed as backsplash galaxies that were once satellites of
another galactic, group or cluster halo but are today a few Mpc away from them.
These interactions, albeit brief, remove the gas and tidally strip the
outskirts of the dark matter haloes of the now quenched seemingly-isolated
UDGs, which are born as star-forming field UDGs occupying dwarf-mass dark
matter haloes. Quiescent UDGs may therefore be found in non-negligible numbers
in filaments and voids, bearing the mark of past interactions as stripped outer
haloes devoid of dark matter and gas compared to dwarfs with similar stellar
content.
