  Observations suggest that the amount of galactic dust in the Universe
decreased by a factor $\sim 2-3$ during the last $\sim 8$ Gyr. However,
cosmological models of galaxy evolution usually struggle to explain this
decrease. Here we use the semi-analytic model (SAM) L-Galaxies2020 to show that
this drop may be reproduced assuming standard prescriptions for dust production
and evolution. We extend the SAM with i) a state-of-the-art dust model which
adopts the two-size approximation and ii) a new disc instability criterion
which triggers bulge and central black hole growth. The model reproduces some
fundamental properties of the local galaxy population, such as the fraction of
spheroid-dominated galaxies and some scaling relations involving dust.
Moreover, the model predicts a galactic dust drop from $z \sim 1 \rightarrow
0$, which becomes closer to the observed one when adopting the new treatment of
disc instabilities. This result is related to the newly implemented
super-massive black hole growth during disc instabilities, which enhances the
quenching of massive galaxies. Consequently, these objects feature a lower gas
and dust content. We provide a census of the contribution of all the processes
affecting the galactic dust content. Accretion is the dominant dust mass growth
process. Destruction by supernovae, astration and ejection by winds have all a
non-negligible role in decreasing the overall dust content in galaxies below $z
\sim 1$. We also discuss predictions concerning extra-galactic dust, confirming
that a sputtering efficiency lower than the canonical one is required to match
the few available observations.
