  In 2005 Kane & Mele[C. L. Kane and E. J. Mele, Phys. Rev. Lett. 95, 226801
(2005)], predicted that at sufficiently low energy, graphene exhibits a
topological state of matter with an energy gap generated by the atomic
spin-orbit interaction. However, this intrinsic gap has not been measured to
this date. In this letter, we exploit the chirality of the low energy states to
resolve this gap. We probe the spin states experimentally, by employing low
temperature microwave excitation in a resistively detected electron spin
resonance on graphene. The structure of the topological bands is reflected in
our transport experiments, where our numerical models allow us to identify the
resonance signatures. We determine the intrinsic spin-orbit bulk gap to be
exactly 42.2 {\mu}eV. Electron-spin resonance experiments can reveal the
competition between the intrinsic spin-orbit coupling and classical Zeeman
energy that arises at low magnetic fields and demonstrate that graphene remains
to be a material with surprising properties.
