Abstract We report on the experimental observation of dark solitons embedded in a stable periodic pulse train emitted by a net normal dispersion cavity fiber laser. Our experimental result shows that in addition of forming in a continuous-wave background, a dark soliton could also be formed in a stable periodic pulse train background. Numerical simulations have well reproduced the experimental result. Moreover, numerical simulations also show that as a result of the periodic pulse train background the formed dark soliton could exhibit a dark pulse envelope that is much broader than its actual pulse width.