The mass-like asymmetry of the quark spectrum in the gluon vacuum is investigated. The meaning of the spectral density for quark states is clarified and its connection with the gluon condensate is established. The concept of a non-perturbative low-energy region is applied to stabilize the low-energy fermion vacuum. The exact equation for the averaged quark spectrum is derived. It reveals the non-perturbative features of the dynamical quark mass.