Plasmon hybridization modes are observed in the extinction spectra of a metal-insulator-metal (MIM) nanodisk array fabricated using nanospherical-lens lithography. Two distinct hybridization modes are observed in this vertically aligned configuration. Theoretical simulation indicates that the bonding mode located at a lower energy level exhibits an antiphase charge distribution and corresponds to the dark plasmon mode. This is vastly different compared to antibonding dark plasmon mode observed in the conventional dimer configuration. The observed mode is tunable over a wide spectral range simply by varying the insulator thickness and the diameters of the MIM nanodisks. Absorption is the dominating extinction process for the dark plasmon, while scattering dominates the bright plasmon mode. The ability to experimentally measure and tune dark plasmon modes using a MIM configuration should catalyze more novel studies that take full advantages of the absorption-dominated dark plasmon mode.