  The sterile neutrino ($N_1$) features in multiple extensions of the standard
model and is a compelling dark matter candidate, especially as the decay of
$N_1$ with mass $m_\mathrm{s}=7.1\mathrm{keV}$ is a possible source for the
unexplained $3.55\mathrm{keV}$ X-ray line reported in galaxy clusters. This
particle will be accessible to the XRISM X-ray mission over the next 12 months.
We revisit the physics behind $N_1$ and the uncertainty in its parameters. We
compare predictions for the $m_{s}=7.1\mathrm{keV}$ $N_1$ mixing angle,
$\sin^2(2\theta)$, and half-mode mass, $M_\mathrm{hm}$, as described in the
$\nu$MSM standard model extension to existing X-ray observations and structure
formation constraints. The strongest available constraints rule out $N_1$ as a
dark matter candidate, and a more optimistic reading of the data prefers
$\sin^2(2\theta)=5\times10^{-11}$ and $M_\mathrm{hm}=3.5\times10^{8}M_{\odot}$.
We highlight that the most promising upcoming opportunity for a detection is to
find a line of velocity dispersion $\sim500\mathrm{kms}^{-1}$ in the Virgo
cluster with XRISM, and then draw up a list of future objects of study to
determine: (i) whether the line is from dark matter generally, and (ii) if from
dark matter, whether that candidate is indeed $N_1$.
