  We investigate the mass spectra of exotic hadrons known as hexaquarks in the
form of dibaryons. We use a phenomenological model based on an extended version
of the G\"ursey-Radicati mass formula for hadrons to include non-charmed
baryons, charmed baryons, and non-strange dibaryons to be able to predict
masses of potential dibaryon states. We perform six numerical fits of this
model to input data for three different sets of masses of baryons and
dibaryons. We find that the model can fit some of the data sets well,
especially the sets including charmed baryons and non-strange dibaryons, and
observe that the predicted mass of one of the dibaryons is close to the
measured mass of the observed hexaquark candidate $d^*(2380)$ reported by the
WASA-at-COSY experiment. The predicted mass of the deuteron is slightly larger
than its measured mass. Finally, for the data sets including charmed baryon and
non-strange dibaryon masses, we find that the predicted masses of potential
dibaryon states are all in the range from 1900 MeV to 3700 MeV.
