  In this work we extend simple top-hat model of structure formation to the
two-component system made of baryonic and dark matter. We use
Harrison-Zeldovich spectrum as the initial condition for the structures and
calculate their evolution up to the present time. While we do not take into
account some complications during the structure formation, such as the merging
of galaxies, however this formalism can give us a qualitative picture from the
formation of structures. We show that in this model small scale structures
evolve faster than the larger ones and it predicts a down-top scenario for the
structure formation. The trend of power spectrum in this model is compatible
with the observations and results in $\sigma_8 \sim 0.8$. This formalism
provides an analytic treatment of structure growth and can easily show the
effect of the cosmological parameters on the formation of the structures. As an
example, the effect of a parameterized dark energy model on the growth of the
structures is investigated.
