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How did small cosmic microwave background fluctuations help galaxies form?

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Small fluctuations in the cosmic microwave background (CMB) represent tiny variations in the density of the early universe, which were essential for the formation of large-scale structures like galaxies and clusters of galaxies.1

These fluctuations originated as quantum fluctuations in mass density during the period of cosmic inflation. Because the early universe was not perfectly uniform, these slight differences in density meant that some regions contained more matter than others. Over time, these denser regions exerted a stronger gravitational pull, attracting surrounding matter and allowing it to condense into protogalaxies.123

The presence of dark matter played a critical role in this process. As a nonbaryonic component, dark matter provided the necessary gravitational scaffolding that allowed these density fluctuations to grow and coalesce into the structures we observe today. Without these initial departures from exact isotropy, the universe would have remained a structureless, uniform distribution of matter.451