From simulations and observations, there is strong evidence that galaxies formed before clusters. This means that small galaxies merged into larger galaxies that would all be attracted toward a common center of mass, called a galaxy cluster. Because there is strong evidence for this, a bottom-up structure is much more favored. This means, CDM seems to be the leading candidate for dark matter and structure formation. This is one of the strongest reasons that CDM is one of the most probable candidates for dark matter. Figure 5 shows the the different possibilities of structure formation using HDM and CDM. We observe that the galaxies are more clumpy than spread out like the image on the right. The CDM image is what we observe on a regular basis, so therefore CDM seems the most likely candidate for dark matter. (1, 2, 3)