Before the Milky Way became the graceful spiral dominating our local cosmos, it appears to have been a messy swarm of thousands of tiny galaxies colliding, merging, and tearing themselves apart across billions of years. The picture comes from a new suite of supercomputer simulations that reconstruct how a Milky Way–type galaxy could grow from a primordial web of small structures into the starry disk we see today.
The work, led by Harley Katz, an assistant professor of astronomy and astrophysics at the University of Chicago, models the first several billion years of our galaxy’s history in unprecedented detail. The team’s simulation follows hot gas as it cools, condenses into stars, and then gets blasted back out by supernova explosions, repeating this cycle over and over as thousands of proto-galaxies interact. Some of those early galaxies are intense star factories, some are ghostly gas clouds, and others are populated mostly by dead stars and black holes, yet together they eventually coalesce into a single dominant system recognizable as the Milky Way’s spiral disk.
This chaotic origin story lines up with the now-standard ΛCDM (“Lambda Cold Dark Matter”) cosmological model, which predicts that dark matter collapses first into small halos that then attract normal matter and form dwarf galaxies—the building blocks of larger systems. Over time, those dwarfs merge hierarchically in a “bottom-up” process to build disk galaxies like the Milky Way, with observations and earlier simulations already hinting that big galaxies should be surrounded by hundreds of smaller companions, most later cannibalized by the main galaxy. Astronomers have long argued that spiral galaxies like ours didn’t take their modern form until roughly 10 billion years ago, after many rounds of mergers and disk-building star formation.
Galaxy-formation sims aren’t new—projects such as the FIRE-based Latte simulations have been tracking Milky Way–mass halos across the universe’s full 13.8-billion-year history, modeling everything from the dark-matter halo to giant molecular clouds and satellite galaxies. What sets Katz’s work apart is its focus on the early, swarm-like phase of a Milky Way analog, pushing resolution and physics to capture the rich diversity of proto-galaxies that feed the final spiral. That level of detail helps researchers test how feedback from stars, gas dynamics, and mergers sculpt the galaxy’s present-day structure, including its stellar halo and surviving dwarf companions.
For anyone who thinks of the Milky Way as a serene, stable backdrop for sci-fi adventures, these simulations reframe our home as the end product of a cosmic battle royale where thousands of contenders fought for survival. The results give theorists a sharper target for interpreting observations of dwarf galaxies, stellar streams, and ancient star populations, and they dovetail with campaigns to watch real galaxies in their youth using deep surveys and studies of the epoch of galaxy formation. As simulation power grows and telescopes continue to probe earlier eras of the universe, expect the origin story of the Milky Way—and the galaxies that host all our favorite space operas—to get even stranger and more detailed.
