  Understanding and controlling the way excitons propagate in solids is a key
for tailoring materials with improved optoelectronic properties. A fundamental
step in this direction is the determination of the exciton energy-momentum
dispersion. Here, thanks to the solution of the parameter-free Bethe- Salpeter
equation (BSE), we draw and explain the exciton energy-momentum map of
hexagonal boron nitride (h-BN) in the first three Brillouin zones. We show that
h-BN displays strong excitonic effects not only in the optical spectra at
vanishing momentum $\mathbf{q}$, as previously reported, but also at large
$\mathbf{q}$. We validate our theoretical predictions by assessing the
calculated exciton map by means of an inelastic x-ray scattering (IXS)
experiment. Moreover, we solve the discrepancies between previous experimental
data and calculations, proving then that the BSE is highly accurate through the
whole momentum range. Therefore, these results put forward the combination BSE
and IXS as the tool of choice for addressing the exciton dynamics in complex
materials.
