In a recent paper we reported dark pulse emission of an all-normal-dispersion-fiber laser [Zhang, Tang, Zhao, and Wu, Phys. Rev. A 80, 045803 (2009)]. However, the formation mechanism of the dark pulse in the laser was unclear due to the limited temporal resolution of the measurement system. Using an improved detection system we have further investigated the phenomenon. We not only experimentally, unambiguously confirmed the existence of dark solitons in the fiber laser, but also identified that the dark pulses observed previously were bunches of the dark solitons. Moreover, we show that the dark soliton formation is a generic feature of the all-normal-dispersion-fiber lasers.