  For years, the extragalactic community has divided galaxies in two distinct
populations. One of them, featuring blue colours, is actively forming stars,
while the other is made up of "red-and-dead" objects with negligible star
formation. Yet, are these galaxies really dead? Here we would like to highlight
that, as previously reported by several independent groups, state-of-the-art
cosmological numerical simulations predict the existence of a large number of
quenched galaxies that have not formed any star over the last few Gyr. In
contrast, observational measurements of large galaxy samples in the nearby
Universe suggest that even the most passive systems still form stars at some
residual level close to $sSFR\sim10^{-12}~\text{yr}^{-1}$. Unfortunately,
extremely low star formation poses a challenge for both approaches. We conclude
that, at present, the fraction of truly dead galaxies is still an important
open question that must be addressed in order to understand galaxy formation
and evolution.
