The idea of a bottom-up formation would be that the small density fluctuations of the early Universe would have been similar in size to small galaxies that we observe today. This theory predicts that the small galaxies would have been gravitationally bound to other small galaxies. These small galaxies would come into contact and eventually merge. The merging of many small galaxies would result in fewer, large galaxies that are the size we observe today. For this to happen, gravity would have to be the controlling factor because gravity is the sole reason the galaxies would merge. This model is more favorable for Cold Dark Matter because the velocities of the CDM would need to be low enough to produce this structure. To produce clumping, you need slow moving and massive particles, so this process can not be done using HDM. Figure 4 shows the process of Bottom-Up Structure Formation. (1, 2, 3)