How Galaxies Form

01 Seeds in the Primordial Universe

The story of galaxy formation begins roughly 380,000 years after the Big Bang, when the universe cooled enough for protons and electrons to combine into neutral hydrogen. This moment — called recombination — released the cosmic microwave background, a snapshot of the universe at its earliest visible stage. In that image, the seeds of all future galaxies are visible as tiny temperature fluctuations: regions where the density of matter was fractionally higher than average.

These density fluctuations originated as quantum fluctuations during cosmic inflation, a fraction of a second after the Big Bang. Inflation stretched microscopic quantum perturbations to macroscopic scales, imprinting a spectrum of density variations across the entire observable universe. The initial density contrast was extraordinarily small — about one part in 100,000. Yet gravity is patient. Over billions of years, regions that started slightly denser pulled in more matter, grew increasingly overdense, and eventually collapsed under their own weight to form structures.

Crucially, the matter that collapsed first was not ordinary atomic matter. It was dark matter. Ordinary matter — baryons — was coupled to the radiation field and could not collapse until after recombination. Dark matter, being electromagnetically inert, was free to collapse much earlier. It began forming gravitationally bound structures — dark matter halos — as early as a few million years after the Big Bang, well before the first stars ignited. These halos would become the scaffolding upon which all galaxies were built.

02 Dark Matter: The Invisible Scaffolding

Dark matter is the architectural framework of every galaxy, yet it emits, absorbs, or reflects no light. It reveals itself only through gravity. The evidence is overwhelming: the rotation curves of spiral galaxies remain flat at large radii rather than declining as Keplerian orbits would predict, implying the presence of an extended, massive, invisible halo. Gravitational lensing — the bending of light by mass — maps dark matter halos directly. The orbital velocities of galaxies within clusters require far more mass than the luminous matter provides. Dark matter accounts for roughly 85 percent of all matter in the universe.

In the standard Lambda-CDM cosmological model, dark matter is cold — meaning its particles moved slowly compared to the speed of light when they decoupled from the early radiation field. This property is critical: cold dark matter can collapse into structures on small scales, enabling the hierarchical formation process in which small halos form first and merge into larger ones. Hot dark matter, such as neutrinos, would have washed out small-scale structure and produced a fundamentally different cosmic web.

Dark matter halos form through a process called gravitational collapse. A region whose density exceeds the cosmic average by a sufficient margin stops expanding with the universe and begins to collapse. As the halo contracts, dark matter particles — collisionless and unable to radiate energy — oscillate within the gravitational potential well. They settle into a roughly virialized structure with a characteristic density profile known as the NFW profile, after Navarro, Frenk, and White, who described it in 1997. The halo's gravitational well is the trap that will capture gas and form a galaxy.

03 Gas Cooling and the First Stars

A dark matter halo alone cannot make a galaxy. It needs baryonic matter — gas. After recombination, hydrogen and helium gas fell into the gravitational wells of dark matter halos. But simply falling in was not enough; the gas had to cool. If the gas remained hot, thermal pressure would resist further collapse, and no stars would form. The ability of gas to radiate away its thermal energy — its cooling rate — determined whether a galaxy could form inside a given halo.

In halos with masses below roughly 100 million solar masses, atomic hydrogen cooling was inefficient. The gas could not shed enough heat to collapse to stellar densities. Only in more massive halos, where the density was high enough for molecular hydrogen to form, could the gas cool sufficiently. Molecular hydrogen acts as a radiator, emitting infrared photons that carry away thermal energy. This is how the first stars — the so-called Population III — ignited, perhaps 100 million years after the Big Bang.

Population III stars were unlike any stars alive today. Composed of pristine hydrogen and helium from the Big Bang, with no heavy elements to cool the gas, they grew to extraordinary masses — potentially hundreds of solar masses. They burned hot, lived fast, and died young. Their supernova explosions — far more powerful than most supernovae today — blasted metals into the surrounding gas, enriching the interstellar medium for the first time. These first heavy elements changed the thermodynamics of gas cooling forever, enabling more efficient star formation in subsequent generations.

04 Hierarchical Merging: Building Bigger From Smaller

The dominant paradigm of galaxy formation is hierarchical merging. In the Lambda-CDM model, structure forms from the bottom up: small dark matter halos collapse first, then merge into progressively larger ones. A galaxy like the Milky Way did not form in a single event — it was assembled over billions of years from hundreds of smaller pieces. The evidence for this process is written in the stellar halo of the Milky Way, which contains the fossilized remnants of dwarf galaxies that were consumed and disrupted.

The merging process drives much of the diversity in galaxy morphology. When gas-rich spiral galaxies merge, the gravitational interaction disturbs their disks, triggers massive bursts of star formation (starburst galaxies), and ultimately produces an elliptical galaxy. The Antennae Galaxies, about 60 million light-years away, show this process in progress: two spirals tearing into each other in a blaze of new star formation. The final product will be a single elliptical galaxy with little remaining gas and an old stellar population.

Mergers also feed supermassive black holes at galactic centers. Gas driven inward by the gravitational disturbance can accrete onto the central black hole, powering quasars — the most luminous objects in the universe. This co-evolution of galaxies and their central black holes is a central theme of modern astrophysics. The mass of a galaxy's central black hole correlates tightly with the velocity dispersion of its stellar bulge, suggesting a link between the two that is still not fully explained.

05 Morphology: Why Galaxies Look Different

Galaxies come in three broad morphological types: spirals, ellipticals, and irregulars. Edwin Hubble introduced this classification in 1926, arranging them in a tuning-fork diagram that he interpreted as an evolutionary sequence. We now know that Hubble's tuning fork does not represent a single evolutionary path — galaxies do not progress from elliptical to spiral — but the classification remains useful.

Spiral galaxies like the Milky Way are disk-dominated systems with ongoing star formation. Their disks formed when gas collapsed conserving its angular momentum, settling into a thin rotating plane. Spiral arms are not rigid structures but density waves — regions where gas compression triggers bursts of star formation, creating the bright blue arms of young stars that characterize spirals. The central bulge contains older stars in more random orbits. Spiral galaxies retain large gas reservoirs and continue forming stars at modest rates.

Elliptical galaxies are the end products of major mergers. When two comparably massive spirals merge, their ordered rotation is converted into random stellar motions, producing a smooth, featureless ellipsoid. The merger consumes or expels most of the gas, so ellipticals typically have little ongoing star formation and are dominated by old, red stellar populations. They range from dwarf ellipticals to the most massive galaxies in the universe — cD galaxies at the centers of galaxy clusters, weighing a trillion solar masses or more. Irregular galaxies — small, chaotic systems without clear structure — are often the result of gravitational disturbances or recent mergers in progress.

06 The Cosmic Web and Large-Scale Structure

Galaxies are not distributed uniformly through space. They trace a vast network — the cosmic web — consisting of dense clusters connected by filaments of galaxies, with vast, nearly empty voids in between. This pattern emerged from the same density fluctuations that seeded individual galaxies, operating on larger scales. The cosmic web is the largest structure in the universe, spanning hundreds of millions of light-years, and it organizes the matter distribution of the entire cosmos.

Galaxy clusters sit at the intersections of filaments — the densest nodes of the cosmic web. A single cluster can contain thousands of galaxies, embedded in a halo of hot X-ray-emitting gas that can total more mass than all the galaxies combined. The gas in clusters, heated to tens of millions of degrees by gravitational collapse, is detectable in X-rays and provides an independent measure of the cluster's total gravitational mass. Cluster mass function — the number of clusters above a given mass — is one of the most powerful cosmological probes, constraining the amplitude of the primordial fluctuation spectrum and the density of dark matter.

The filaments themselves are rivers of gas and dark matter, channeling material into clusters and feeding galaxy growth. Simulations of structure formation — N-body hydrodynamics computations that track both dark matter and baryonic gas — reproduce the cosmic web with remarkable fidelity. The IllustrisTNG and EAGLE simulations, each involving billions of particles, show how the web forms and how galaxies evolve within it. These simulations are the primary laboratory for testing theories of galaxy formation against observation.

07 What the James Webb Telescope Is Revealing

The James Webb Space Telescope, launched in 2021, has transformed the study of early galaxy formation. Its infrared sensitivity allows it to detect light from galaxies that has been redshifted by the expansion of the universe into wavelengths beyond the visible spectrum. With JWST, astronomers can observe galaxies as they were when the universe was less than 500 million years old — closer to the first galaxies than any previous telescope could reach.

Early JWST results have been surprising. Some galaxies observed at very high redshift appear more massive, more structured, and more metal-enriched than the standard model predicted. This tension has prompted debate: whether the first galaxies formed faster than expected, whether early star formation was more efficient, or whether cosmological parameters need revision. The data are still accumulating, and the theoretical implications are being actively worked out.

What is clear is that galaxy formation was underway far earlier than we could confirm before. The first galaxies were not gentle, isolated objects — they were intense, rapidly evolving systems forming stars at rates that dwarf anything in the local universe. Their light, stretched across 13 billion years of cosmic expansion, carries the record of the first structures that ever existed. The cosmic web we inhabit today, and the galaxy we call home, began in their violent birth.