How do density fluctuations lead to galaxies and galaxy clusters?

In the early universe, structure emerged from primordial density fluctuations—small variations in the distribution of matter. As the universe expanded and cooled, these fluctuations allowed matter to condense into small clouds known as protogalaxies.1

These regions of higher density possessed stronger gravitational fields, enabling them to collapse and form the building blocks of galaxies. Crucially, this process was driven by dark matter, which makes up the vast majority of the matter in the universe. Because this nonbaryonic dark matter is "cold" (nonrelativistic and slow-moving), it acted as a gravitational backbone, facilitating the coalescence of galaxies and larger structures like clusters of galaxies.123

Over time, these structures continued to evolve through the merger and accretion of smaller entities. Computer simulations, known as n-body simulations, demonstrate that the universe developed as a network-like arrangement where massive galaxies formed at the intersections of these strands, often surrounded by smaller dwarf galaxies.1