  The cold dark matter (DM) model predicts that every galaxy contains thousands
of DM subhalos; almost all other DM models include a physical process that
smooths away the subhalos. The subhalos are invisible, but could be detected
via strong gravitational lensing, if they lie on the line of sight to a
multiply-imaged background source, and perturb its apparent shape. We present a
predominantly automated strong lens analysis framework, and scan for DM
subhalos in Hubble Space Telescope imaging of 54 strong lenses. We identify
five DM subhalo candidates, including two especially compelling candidates (one
previously known in SLACS0946+1006) where a subhalo is favoured after all of
our tests for systematics. We find that the detectability of subhalos depends
upon the assumed parametric form for the lens galaxy's mass distribution,
especially its degree of azimuthal freedom. Using separate components for dark
matter and stellar mass reveals two DM subhalo candidates and removes four
false-positives compared to the single power-law mass model that is common in
the literature. We identify 45 lenses without substructures, the number of
which is key to statistical tests able to rule out models of e.g. warm or
self-interacting DM. Our full analysis results are available at
https://github.com/Jammy2211/autolens_subhalo.
