The dibaryon resonance $d^*(2380)$ with $I(J^P) = 0(3^+)$ --- first
observed in the double-pionic fusion to the deuteron --- has meanwhile been
detected in all relevant two-pion production channels by incident
neutron-proton collisions. In addition, its resonance pole has been revealed
in neutron-proton scattering.
Theoretical calculations describe this state either as a compact hexaquark
system or a dilute molecular-like object. Whereas the $d^*$ decay into two-pion
channels does not discriminate between these two scenarios, the decay into
single-pion channels is very discriminatory. In the hexaquark case this decay is
heavily suppressed with a branching of less than 1$\%$. In the molecular-like
case a branching of as much as 18$\%$ is expected.
In order to clarify this situation we have measured the isoscalar single-pion
production in the energy region of d*(2380). As a result we find no evidence
for such a decay with an upper limit of smaller than 9$\%$. This is in support
of the hexaquark interpretation -- at least as the dominant configuration,
possibly surrounded by a cloud of molecular-like configurations. Other
dibaryon candidates are currently under investigation.