  The Standard Model of particle physics describes the properties of elementary
particles and their interactions remarkably well, but in particular does not
account for dark matter. Isotope-shift spectroscopy is a sensitive probe of
fifth forces and new particles that illuminate the dark matter sector. This
method sets bounds on new bosons that couple neutrons and electrons with masses
in the keV/c2 to MeV/c2 range. With increasing spectroscopic precision, such
searches are limited by uncertainties of isotope masses and the understanding
of nuclear structure. Here, we report on high-precision mass-ratio and
isotope-shift measurements of the ytterbium isotopes $^{168,170,172,174,176}$Yb
that exceed previous measurements by up to two orders of magnitude. From these
measurements, we extract higher-order changes in the nuclear charge
distribution along the Yb isotope chain and use these to benchmark novel ab
initio calculations. Our measurements set new bounds on the existence of the
proposed boson.
