  We present here results from the first-ever search for dark photon dark
matter that could have coupled to baryons in LISA Pathfinder, the technology
demonstrator for a space-based gravitational-wave antenna. After analyzing
approximately three months of data taken by LISA Pathfinder in the frequency
range $[2\times 10^{-5},5]$ Hz, corresponding to dark photon masses of
$[8\times 10^{-20},2\times 10^{-14}]$ eV/$c^2$, we find no evidence of a
dark-matter signal, and set upper limits on the strength of the dark
photon/baryon coupling. To perform this search, we leveraged methods that
search for quasi-monochromatic gravitational-wave signals in ground-based
interferometers, and are robust against non-Gaussianities and gaps in the data.
Our work therefore represents a proof-of-concept test of search methods in LISA
to find persistent, quasi-monochromatic signals, and shows our ability to
handle non-Guassian artifacts and gaps while maintaining good sensitivity
compared to the optimal matched filter. The results also indicate that these
methods can be powerful tools in LISA to not only find dark matter, but also
look for other persistent signals from e.g. intermediate-mass black hole
inspirals and galactic white dwarf binaries.
