In this talk, I will introduce a beautiful and rich band structure in 2D materials, consisting of a flat valence and a flat conduction band around the Fermi level, named yin-yang flat bands (FBs) characterized with opposite chirality. I will first discuss the construction of the yin-yang FBs based on line-graph theorem and its extension to multiple atoms and orbitals per lattice site in 2D tight-binding lattice models. I will then discuss single exciton formation between the yin-yang FBs, as exemplified in a semiconducting superatomic graphene. Based on density-functional-theory calculations combined with many-body GW and Bethe-Salpeter equation, we shown that massive carriers in FBs with highly localized electron and hole wave functions significantly reduce the screening and enhance the exchange interaction, leading to an unusually large triplet exciton binding energy (∼1.1 eV) exceeding the GW band gap by ∼0.2 eV. The negative exciton formation energy, indicating a spontaneous instability in the band structure toward excitonic insulator (EI) state, provides a necessary but not yet a sufficient condition for Bose-Einstein condensation (BEC) of excitons. Using exact diagonalization of many-exciton Hamiltonian, we further show that the triplet EI state is stable for exciton densities up to complete population inversion between the two FBs. Importantly, analyses of many-exciton wavefunctions and calculation of reduced density matrices confirm the quantum coherence of excitonic BEC. We elucidate that it is the presence and contribution of the yin-yang FBs to the excitonic levels that stabilize the BEC state. Our results demonstrate that differing from parabolic bands, FBs provide a unique platform for material realization of spinor BEC and spin superfluidity.