  We present the most distant detection of cosmic voids ($z \sim 2.3$) and the
first detection of three-dimensional voids in the Lyman-$\alpha$ forest. We
used a 3D tomographic map of the absorption with effective comoving spatial
resolution of $2.5\,h^{-1}\mathrm{Mpc}$ and volume of $3.15\times
10^5\,h^{-3}\mathrm{Mpc}^3$, which was reconstructed from moderate-resolution
Keck-I/LRIS spectra of 240 background Lyman-break galaxies and quasars in a
$0.16\,\mathrm{deg}^2$ footprint in the COSMOS field. Voids were detected using
a spherical overdensity finder calibrated from hydrodynamical simulations of
the intergalactic medium. This allows us to identify voids in the IGM
corresponding to voids in the underlying matter density field, yielding a
consistent volume fraction of voids in both data (19.5%) and simulations
(18.2%). We fit excursion set models to the void radius function and compare
the radially-averaged stacked profiles of large voids ($r > 5$ $h^{-1}$ Mpc) to
stacked voids in mock observations and the simulated density field. Comparing
with 432 coeval galaxies with spectroscopic redshifts in the same volume as the
tomographic map, we find that the tomography-identified voids are underdense in
galaxies by 5.95$\sigma$ compared to random cells.
