  The 3.5 keV line is a purported emission line observed in galaxies, galaxy
clusters, and the Milky Way whose origin is inconsistent with known atomic
transitions and has previously been suggested to arise from dark matter decay.
We systematically re-examine the bulk of the evidence for the 3.5 keV line,
attempting to reproduce six previous analyses that found evidence for the line.
Surprisingly, we only reproduce one of the analyses; in the other five we find
no significant evidence for a 3.5 keV line when following the described
analysis procedures on the original data sets. For example, previous results
claimed 4$\sigma$ evidence for a 3.5 keV line from the Perseus cluster; we
dispute this claim, finding no evidence for a 3.5 keV line. We find evidence
for background mismodeling in multiple analyses. We show that analyzing these
data in narrower energy windows diminishes the effects of mismodeling but
returns no evidence for a 3.5 keV line. We conclude that there is little robust
evidence for the existence of the 3.5 keV line. Some of the discrepancy of our
results from those of the original works may be due to the earlier reliance on
local optimizers, which we demonstrate can lead to incorrect results. For ease
of reproducibility, all code and data are publicly available.
