  We confirm our previous prediction of a $d^*$ state with $I(J^P)=0(3^+)$
[Phys. Rev. C 60, 045203 (1999)] and report for the first time based on a
microscopic calculation that $d^*$ is a hexaquark dominated exotic state as it
has about 2/3 hidden color (CC) configurations. By performing a more elaborate
dynamical coupled-channels investigation of the $\Delta\Delta$-CC system within
the framework of resonating group method (RGM) in a chiral quark model, we
found that the $d^*$ state has a mass of about 2.38-2.42 GeV, a
root-mean-square radius (RMS) of 0.76-0.88 fm, and a CC fraction of 66%-68%.
The last ensures that the $d^*$ has a rather narrow width which, together with
the quantum numbers and our calculated mass, is consistent with the newly
observed resonance-like structure ($M\approx 2380$ MeV, $\Gamma\approx 70$ MeV)
in double-pionic fusion reactions reported by WASA-at-COSY Collaboration.
