  Two of the most pressing questions in physics are the microscopic nature of
the dark matter that comprises 84% of the mass in the universe and the absence
of a neutron electric dipole moment. These questions would be resolved by the
existence of a hypothetical particle known as the quantum chromodynamics (QCD)
axion. In this work, we probe the hypothesis that axions constitute dark
matter, using the ABRACADABRA-10cm experiment in a broadband configuration,
with world-leading sensitivity. We find no significant evidence for axions, and
we present 95% upper limits on the axion-photon coupling down to the
world-leading level $g_{a\gamma\gamma}<3.2 \times10^{-11}$ GeV$^{-1}$,
representing one of the most sensitive searches for axions in the 0.41 - 8.27
neV mass range. Our work paves a direct path for future experiments capable of
confirming or excluding the hypothesis that dark matter is a QCD axion in the
mass range motivated by String Theory and Grand Unified Theories.
