As we stated earlier most cosmological models today differ primarily in their total density and dark matter content. With few exceptions they are based upon nearly scale-invaraint adiabatic gaussian initial fluctuations, such as produced with inflation. The first major distinction is between cold dark matter and hot dark matter. In the former scenario there is power over a wide range of physical scales, and therefore on small scales, and structure forms beginning with small objects which then merge to form ever larger structures. Structure forms first large scale objects which fragment into smaller objects.
Small scales are to the right on the figure.
Notice that the CDM model has much more power on small scales than the HDM model. Once we have decided on CDM, the next biggest question is the total matter density in the universe. We could have a high-density universe of mainly CDM, or a low density universe where the bulk of the energy density is in a cosmological constant, curvature or some other form. In the low-density universe, matter ceases to dominate the expansion rate at some point, and thereafter structure stops growing. There is not enough gravitational pull to overcome the expansion of the universe and keep the structures growing.