  As the largest gravitationally bound objects in the Universe, galaxy clusters
have provided the first piece of evidence for the presence of dark matter and
may be suitable targets for indirect dark matter searches. Among various
signals, the GeV-TeV $\gamma$-ray line has been taken as the smoking-gun signal
of the dark matter annihilation/decay since no known astrophysical/physical
process(es) could generate such a peculiar spectrum. With 15.5 years of
Fermi-LAT P8R3 publicly available data, we search for the $\gamma$-ray line
emission in the directions of a group of 13 nearby massive galaxy clusters with
an unbinned likelihood analysis. A $\gamma$-ray line signal at $\sim 43.2$ GeV
has a net TS value of $\approx 30$ if we only take into account the data in the
directions of Virgo, Fornax and Ophiuchus clusters, three massive clusters with
the highest J-factors expected to generate the dark matter annihilation signal.
The signal still presents when the data of other 10 nearby massive clusters
have also been included, though the TS value decreases to $\approx 21$ likely
because of their lower signal-to-noise ratios. The absence of this signal in
the inner Galaxy disfavors both the instrumental effect and the canonical dark
matter annihilation interpretation, and a more sophisticated dark matter model
or very peculiar astrophysical scenario might be needed. This $\gamma$-ray line
signal, if intrinsic, could be unambiguously verified by the Very Large Area
$\gamma$-ray Space Telescope in its first two years of performance.
