  The Universe is mostly composed of large and relatively empty domains known
as cosmic voids, whereas its matter content is predominantly distributed along
their boundaries. The remaining material inside them, either dark or luminous
matter, is attracted to these boundaries and causes voids to expand faster and
to grow emptier over time. Using the distribution of galaxies centered on voids
identified in the Sloan Digital Sky Survey and adopting minimal assumptions on
the statistical motion of these galaxies, we constrain the average matter
content $\Omega_\mathrm{m}=0.281\pm0.031$ in the Universe today, as well as the
linear growth rate of structure $f/b=0.417\pm0.089$ at median redshift
$\bar{z}=0.57$, where $b$ is the galaxy bias ($68\%$ C.L.). These values
originate from a percent-level measurement of the anisotropic distortion in the
void-galaxy cross-correlation function, $\varepsilon = 1.003\pm0.012$, and are
robust to consistency tests with bootstraps of the data and simulated mock
catalogs within an additional systematic uncertainty of half that size. They
surpass (and are complementary to) existing constraints by unlocking
cosmological information on smaller scales through an accurate model of
nonlinear clustering and dynamics in void environments. As such, our analysis
furnishes a powerful probe of deviations from Einstein's general relativity in
the low-density regime which has largely remained untested so far. We find no
evidence for such deviations in the data at hand.
