This work is based on predicting damage density in graphite/epoxy polymer matrix composite materials (PMC) under hygrothermal aging condition. A coupled Forward-Backward Stochastic Differential Equation (FBSDE) was proposed as a mathematical model to predict number density of damages. The uncertainty in damage nucleation and annihilation rate was captured by proposing these two terms as "Brownian motion with drift". In order to verify the proposed model, a rigorous quantitative damage analysis was carried out on graphite/epoxy specimens. The specimens were kept under certain hygrothermal condition. The damages were classified based on the criteria of their size. Finally, the predicted damage density from the proposed stochastic model was verified against experimental data. The proposed model showed good agreement with experimental observation.