In order to understand the types of particles that might explain dark matter, it is first important to understand what we observe in the present Universe and how that influences our ideas of the formation and structure of dark matter. There are two possible ways that the early Universe could have formed to create structures that we see today. The two different models are the Top-Down Structure Formation and the Bottom-Up Structure Formation. (1, 2, 3)
The idea of top-down structure formation is in the early Universe radiation smoothed out the the small matter density fluctuation to create a large, thin, disk structure. In order for this to happen, the dark matter particles would have to be moving at relativistic speeds. As time progressed, the structure fragmented into many “pancake” structures. These structures would have been very large and than the pancakes would have fragmented into smaller sized lumps of matter, or galaxies. This process is more favorable for Hot Dark Matter because the speeds of the particles need to be relativistic to create the smooth structure. This formation is basically is just the fragmentation of a large clump into smaller clumps. Figure 3 shows the process for Top-Down Structure Formation. (1, 2, 3)