As a result, masses of baryonic matter started to condense within cold dark matter halos.[89][90] These primordial structures would eventually become the galaxies we see today.
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The detailed process by which such early galaxy formation occurred is a  major open question in astronomy. Theories could be divided into two  categories: top-down and bottom-up. In top-down theories (such as the  Eggen–Lynden-Bell–Sandage [ELS] model), protogalaxies form in a  large-scale simultaneous collapse lasting about one hundred million  years.[94] In bottom-up theories (such as the Searle-Zinn [SZ] model), small structures such as globular clusters form first, and then a number of such bodies accrete to form a larger galaxy.[95]