  Almost 50% of galaxies in the local Universe are in clusters or groups
coexisting with both hot and cold gas components. In the present study, we
observationally probed the cold-gas content of X-ray-selected massive galaxy
clusters with spectroscopic redshift measured from the SDSS/SPIDERS survey.
This paper focuses on the most massive structures: galaxy clusters with a mean
mass of M$_{500c}$ = 2.7$\times 10^{14}$ M$_{\odot}$. We used a large number of
background quasar optical spectra from SDSS DR16 to probe the diffuse
T$=$10$^4$K gas in their intracluster medium. We first analysed a sample of
spectra with known MgII absorbers, and then blindly stacked about 16,000
archival spectra at the redshifts of the foreground galaxy clusters. We
tentatively ($3.7 \sigma$ significance) detect MgII in the clusters with an
equivalent width EW(MgII $\lambda$2796) of 0.056$\pm$0.015 \r{A}, corresponding
to a column density of log [N(MgII)/cm$^{-2}$]=12.12$\pm0.1$. We tested our
methodology by generating 22,000 mock SDSS spectra with MgII absorbers from
TNG50 cosmological magnetohydrodynamical simulations, combining
photo-ionisation modelling and ray tracing. We also performed bootstrapping
stacking at different cluster redshifts and stacked quasar spectra with no
intervening clusters in the line of sight to measure the significance of our
detection. These results are in line with the findings of recent, similar
observational studies but challenge predictions from TNG simulations. Together,
our findings indicate that large amounts of cold gas may be found in the most
massive structures of the Universe.
