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Only long after that , starting with small mass scales and advancing over time to larger mass scales , do the perturbations actually collapse to form ( for example ) galaxies or clusters of galaxies , in so-called hierarchical structure formation ( see Physical cosmology ) .
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Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
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