By the end of the Particle Age, the universe was filled with a thin, spread-out gas. While it looked mostly the same everywhere, there were tiny patches that were slightly denser or more crowded than others. In these “crowded” spots, the gas began to clump together—pulled in by the gravity of dark matter. As these clouds of gas collapsed under their own weight, they eventually ignited to form the very first stars and galaxies. You can see this transformation in the simulation below.

The Millennium Simulation, pioneered in 2005 by Volker Springel and the Virgo Consortium, serves as a landmark computational model for visualizing the gravitational evolution of the universe. By tracking cold dark matter from a redshift of 20—when the cosmos was only 180 million years old—to the present day, the simulation reveals the emergence of a “cosmic web” across vast spatial scales. This timelapse created from the simulation shown above demonstrates how gravity relentlessly pulls matter into a complex network of sweeping filaments and ultra-dense nodes, providing a scaffolding to the visible distribution of galaxies.

Empirical observations from the Sloan Digital Sky Survey and the 2dF Galaxy Redshift Survey confirm that the universe is organized into a non-random, sponge-like architecture rather than a chaotic scattering of matter. In this grand design, galaxies and clusters align along interconnected strings and two-dimensional sheets that wrap around enormous cosmic voids. These massive features, such as the Sloan Great Wall, can span hundreds of millions of ly, creating a structural resemblance to neural pathways or the interconnected lights of a global city seen from space.

This colossal scaffold is essentially a magnified fossil of the infant universe, where visible galaxies formed as gas condensed within the gravitational wells of dark matter halos. Modern cosmological models indicate that these macroscopic structures originated from microscopic quantum fluctuations present in the early moments of the Big Bang. During the epoch of inflation, these tiny density variations were stretched to astronomical proportions, setting the blueprint for the interconnected web of matter that defines our observable universe today.

In this striking timelapse of Dhaka from 1984 to 2022, we witness the explosive expansion of a city mirroring the very formation of the cosmic web. During the universe’s Epoch of Reionization, the first galaxies ignited, and their fierce ultraviolet radiation reached out to destroy the surrounding intergalactic neutral hydrogen, leaving behind a fundamentally altered cosmos.

Allegorically, this satellite imagery reveals a similar, localized epoch. Here, sprawling human-made buildings act as the newly formed galaxies. Expanding relentlessly outward in stark shades of grey, these urban clusters swallow and “ionize” Dhaka’s natural green, destroying the lush ecological fabric that once defined the landscape.

Nowhere is this more evident than along the Buriganga. As the grey, galaxy-like concrete pushed ever closer to the water, the river’s green banks were consumed. Stripped of nature, the stark grey boundaries now distinctly outline the river’s winding path from space. Look closely at the newly pronounced curves of the Buriganga—the striking shape forms the clear silhouette of a human face in profile. It is a profound, almost poetic cosmic irony: the landscape itself revealing the literal face of the culprit responsible for the destruction of Dhaka’s natural equilibrium.

Galaxies evolve probably through a “bottom-up” process known as hierarchical merging, where gravity continuously draws smaller pregalactic fragments and dwarf galaxies together to build progressively larger structures. Because the early universe was much denser, these collisions and mergers were frequent, acting as the primary engine driving galactic evolution. When galaxies undergo close encounters or direct collisions, the rapidly varying gravitational forces severely disrupt their original architectures, compressing immense clouds of interstellar gas. This compression frequently triggers explosive, galaxy-wide episodes of stellar birth, creating what are known as starburst galaxies. A spectacular example of this is the Antennae galaxies (pictured above), where two colliding spiral galaxies have drawn out long, sweeping tidal tails of matter and spawned thousands of young, bright “super star clusters” in the wake of violent shock waves. Similar long tidal tails of stars and gas can be seen in another merging system known as the Mice (NGC 4676).

The specific nature of these collisions dictates how a galaxy changes from one type to another on the Hubble sequence. When two massive, comparably sized spiral galaxies undergo a “major merger,” the violent gravitational encounter typically destroys their fragile disks and spiral arms, scrambling the stellar orbits and ejecting much of the gas to ultimately leave behind a featureless elliptical galaxy. Conversely, during a “minor merger,” a large spiral can absorb a much smaller dwarf companion while preserving its overall structure, which is likely how our own Milky Way grew over time. The cosmos is filled with evidence of these ongoing transformations: the Whirlpool Galaxy (pictured above) displays pronounced spiral arms drawn out by a gravitational encounter with a smaller companion, while the active elliptical galaxy Centaurus A features a thick, irregular dust lane resulting from a past collision with a spiral galaxy. Even our Milky Way is currently cannibalizing the Sagittarius dwarf galaxy, and is on a collision course with the Andromeda Galaxy.