  We split the total matter fluid into a bound (halo) component and an unbound
(free particles) fluid component that is accreted by the halos. We adopt a
different framework that treats the structure formation problem as a
gravitational interaction between these virialised cold dark matter halos and
the unbound inter-halo cold dark matter (and cold baryon) particles. This
interaction involves in general an exchange of energy and momentum during the
accretion process. We then explore the evolution of the average matter density
and of large-scale structure formation, using a simplified phenomenological
model that is based on results from extended Press-Schechter and N-body
simulations. At high redshifts most matter is in diffuse form and is not part
of the haloes. As particles are accreted by the virialised halos, the particle
number density decreases and that of the bound matter increases. We also
present a general analysis of the background and linear perturbations for the
interacting fluids, showing in detail the energy and momentum exchange terms.
