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.
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.
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