  Dark matter evolution during the process of cosmological structure formation
can be described in terms of a one-particle irreducible effective action at a
characteristic scale $k_m$ and a loop expansion below this scale, based on the
effective propagators and vertices. We calculate the form of the effective
vertices and compute the bispectrum of density perturbations within a one-loop
approximation. We find that the effective vertices play a subdominant role as
compared to the effective viscosity and sound velocity that modify the
(inverse) propagators. For the bispectrum we reproduce the results of standard
perturbation theory in the range where it is applicable, and find a slightly
improved agreement with N-body simulations at larger wavenumbers.
