Recently, a new model of Galaxy formation has been proposed by Carvalho and Macedo, in several works. They have studied the Jeans instability for a two component (baryonic and dark matter) self-gravitating fluid in the non-relativistic approximation and
embedded in a flat Einstein-de Sitter hot dark matter dominated Universe. They find that a resonance occurs, giving rise to a large peak in the amplitude of the collapse mode of the baryonic component. The corresponding critical mass scale at the time of
recombination is of the order of a typical galaxy for hot dark matter particles with a mass of 30 eV. In this work, we shall change the sufficient mathematical condition for the existence of the baryonic resonance by adopting just tye opposite physical hy
potheses. That is, by assuming a cold dark matter non-dominated open Universe, we also obtain the resonance and hence galaxy formation. However, this study in a open expanding Universe is more complicated that in the flat case.