  We experimentally study magnetic resonances in the helical and conical
magnetic phases of the chiral magnetic insulator Cu$_2$OSeO$_3$ at the
temperature $T$=5 K. Using a broadband microwave spectroscopy technique based
on vector network analysis, we identify three distinct sets of helimagnon
resonances in the frequency range 2 GHz $\leq f \leq$ 20 GHz. The extracted
resonance frequencies are in accordance with calculations of the helimagnon
bandstructure found in an intrinsic chiral magnonic crystal. The periodic
modulation of the equilibrium spin direction that leads to the formation of the
magnonic crystal is a direct consequence of the chiral magnetic ordering caused
by the Dzyaloshinskii-Moriya interaction. The opening of magnon band-gaps
allows for excitation of helimagnons with wave vectors that are multiples of
the spiral wave vector.
