How Galaxies Form: Dark Matter, Cosmic Webs, and the Milky Way's Origins

What Is a Galaxy?

A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas and dust, and dark matter. Galaxies range in size from dwarf galaxies containing a few hundred million stars to giant elliptical galaxies with trillions of stars. The observable universe contains an estimated 2 trillion galaxies. Galaxies are not uniformly distributed in space — they cluster into groups and clusters, which themselves assemble into superclusters connected by filaments of gas and dark matter, with vast voids between them. This large-scale structure is known as the cosmic web.

The study of galaxy formation and evolution is one of the most active frontiers in modern astronomy. Telescopes like the James Webb Space Telescope (JWST), launched in 2021, are now observing galaxies as they appeared in the first few hundred million years after the Big Bang, allowing astronomers to directly observe the early stages of galaxy formation for the first time.

The Role of Dark Matter

Galaxy formation begins not with stars and gas, but with dark matter. In the standard cosmological model (Lambda-CDM), tiny density fluctuations in the distribution of dark matter in the early universe — seeded by quantum fluctuations during cosmic inflation — grew under gravity. Dark matter clumped first, forming gravitational potential wells called dark matter halos. Ordinary matter (gas) fell into these halos, cooled, and eventually formed stars and galaxies.

Dark matter halos are thus the scaffolding on which galaxies are built. Simulations of cosmic structure formation using only gravity and known physics for dark matter reproduce the observed large-scale structure of the universe with remarkable fidelity — the filaments, clusters, and voids of the cosmic web emerge naturally. Without dark matter, this structure cannot be explained. The smallest halos formed first, then merged hierarchically into larger and larger structures — the so-called bottom-up (or hierarchical) model of structure formation.

Types of Galaxies

Edwin Hubble introduced the first widely used galaxy classification in the 1920s, dividing galaxies into ellipticals, spirals, and irregulars. This morphological scheme, updated as the Hubble tuning fork diagram, remains the standard framework despite its limitations.

Spiral galaxies have a central bulge of older stars surrounded by a flat disk containing spiral arms of younger stars, gas, and dust. The spiral arms are regions of active star formation, traced by hot blue stars and glowing nebulae. The Milky Way and the Andromeda Galaxy (M31) are both large spiral galaxies. Barred spiral galaxies — the most common type — have a bar-shaped structure of stars running through the central bulge.

Elliptical galaxies range from spherical to elongated shapes, containing mostly old, red stars with little gas or dust, and minimal current star formation. Giant ellipticals, found at the centers of galaxy clusters, are among the most massive objects in the universe. They are thought to have formed largely through the mergers of smaller galaxies.

Irregular galaxies lack a well-defined shape, often because they have been disturbed by gravitational interactions with neighbors. The Large and Small Magellanic Clouds — small irregular satellite galaxies of the Milky Way — are examples visible to the naked eye from the southern hemisphere.

The Milky Way: Our Home Galaxy

The Milky Way is a barred spiral galaxy approximately 100,000 light-years in diameter, containing an estimated 200-400 billion stars and at least as many planets. The Sun is located about 26,000 light-years from the galactic center, in a relatively minor spiral arm called the Orion Arm. The galactic center harbors a supermassive black hole — Sagittarius A* — with a mass of approximately 4 million solar masses, whose existence was confirmed in 2022 by the first direct imaging of its shadow by the Event Horizon Telescope Collaboration.

The Milky Way formed roughly 13 billion years ago, shortly after the Big Bang, from the merging of smaller proto-galactic fragments. Its stellar population records this history: the oldest stars in the galactic halo formed in the earliest proto-galactic mergers and are metal-poor (containing few elements heavier than helium, because heavy elements had not yet been synthesized in stars); younger stars in the disk are increasingly metal-rich, containing elements forged in previous generations of stars.

Galaxy Mergers and the Future of the Milky Way

Galaxies are not static objects — they merge and interact over cosmic timescales. Galaxy mergers are violent events that can transform the morphology of both galaxies, trigger intense bursts of star formation, feed supermassive black holes at galactic centers, and ultimately produce larger, rounder galaxies. The beautiful tidal tails visible in merging galaxy pairs (such as the Antennae Galaxies) are streams of stars and gas stripped by gravitational interactions.

The Milky Way is itself approaching a collision. The Andromeda Galaxy is moving toward us at roughly 110 km/s and will collide with the Milky Way in approximately 4.5 billion years. The merger is expected to transform both spiral galaxies into a large elliptical, a process astronomers have nicknamed Milkomeda. Despite the collision of entire galaxies, direct stellar collisions will be extraordinarily rare because of the vast distances between stars.

Active Galactic Nuclei

Some galaxies host extraordinarily luminous central regions called active galactic nuclei (AGN), powered by supermassive black holes actively accreting gas and dust. The infalling material forms an accretion disk that heats to millions of degrees and radiates across the electromagnetic spectrum, sometimes outshining the entire rest of the galaxy. Quasars — the most luminous AGN — were first identified as star-like radio sources and are now understood to be the extremely energetic centers of distant, young galaxies. Understanding AGN feedback — the jets and radiation pressure from active black holes that heat and expel gas from the host galaxy, regulating star formation — is one of the central problems of modern galaxy evolution research.