  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 significances for the narrow and broad peaks are
5.3$\sigma$ and 3.5$\sigma$, respectively. This behavior of the photon energy
spectrum is interpreted as a signature of the exotic dibaryon resonance
$d^\star_1$ with a mass of about 1956 MeV 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.
