  The energy spectrum for high energy $\gamma$-rays ($E_\gamma \geq 10$ MeV)
from the process $pp \to \gamma \gamma X$ emitted at $90^0$ in the laboratory
frame has been measured at an energy below the pion production threshold,
namely, at 216 MeV. The resulting photon energy spectrum extracted from $\gamma
- \gamma$ coincidence events consists of a narrow peak at a photon energy of
about 24 MeV and a relatively broad peak in the energy range of (50 - 70) MeV.
The statistical significance for both the narrow and broad peaks exceeds
5$\sigma$. This behavior of the photon energy spectrum is interpreted as a
signature of the exotic dibaryon resonance $d^\star_1$ which is assumed to be
formed in the radiative process $pp \to \gamma d^\star_1$ followed by its
electromagnetic decay via the $d^\star_1 \to pp \gamma$ mode. The experimental
spectrum is compared with those obtained by means of Monte Carlo simulations.
