  The nature of dark matter, the invisible substance making up over $80\%$ of
the matter in the Universe, is one of the most fundamental mysteries of modern
physics. Ultralight bosons such as axions, axion-like particles or dark photons
could make up most of the dark matter. Couplings between such bosons and
nuclear spins may enable their direct detection via nuclear magnetic resonance
(NMR) spectroscopy: as nuclear spins move through the galactic dark-matter
halo, they couple to dark-matter and behave as if they were in an oscillating
magnetic field, generating a dark-matter-driven NMR signal. As part of the
Cosmic Axion Spin Precession Experiment (CASPEr), an NMR-based dark-matter
search, we use ultralow-field NMR to probe the axion-fermion "wind" coupling
and dark-photon couplings to nuclear spins. No dark matter signal was detected
above background, establishing new experimental bounds for dark-matter bosons
with masses ranging from $1.8\times 10^{-16}$ to $7.8\times 10^{-14}$ eV.
