  In this work we perform electron energy-loss spectroscopy (EELS) of
freestanding graphene with high energy and momentum resolution to disentangle
the quasielastic scattering from the excitation gap of Dirac electrons close to
the optical limit. We show the importance of many-body effects on electronic
excitations at finite transferred momentum by comparing measured EELS with ab
initio calculations at increasing levels of theory. Quasi-particle corrections
and excitonic effects are addressed within the GW approximation and
Bethe-Salpeter equation, respectively. Both effects are essential in the
description of the EEL spectra to obtain a quantitative agreement with
experiments, with the position, dispersion, and shape of both the excitation
gap and the $\pi$ plasmon being significantly affected by excitonic effects.
