Top-Down Scenario:

Structure could have formed in one of two sequences: either large structures the size of galaxy clusters formed first, than latter fragmented into galaxies, or dwarf galaxies formed first, than merged to produce larger galaxies and galaxy clusters.

The former sequence is called the top-down scenario, and is based on the principle that radiation smoothed out the matter density fluctuations to produce large pancakes. These pancakes accrete matter after recombination and grow until they collapse and fragment into galaxies.

This scenario has the advantage of predicting that there should be large sheets of galaxies with low density voids between the sheets. Clusters of galaxies form where the sheets intersect.

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Bottom-Up Scenario:

The competing scenario is one where galaxies form first and merge into clusters, called the bottom-up scenario. In this scenario, the density enhancements at the time of recombination were close to the size of small galaxies today. These enhancements collapsed from self-gravity into dwarf galaxies.

Once the small galaxies are formed, they attract each other by gravity and merge to form larger galaxies. The galaxies can then, by gravity, cluster together to form filaments and clusters. Thus, gravity is the mechanism to form larger and larger structures.

(http://abyss.uoregon.edu/~js/ast123/lectures/lec24.html)