Event: Formation and Shape of Galaxies
Date: ~1–3 billion years after the Big Bang

“There is geometry in the humming of the strings, there is music in the spacing of the spheres.”
-Pythagoras

Dear Human,

Before there were galaxies, there were fragments.

In the early universe, matter did not spread evenly. Tiny fluctuations in the density of gas — echoes from the Big Bang itself — gave rise to clumps. These clumps grew under the pull of gravity, merging into clouds, then into swarms, and finally into galaxies. But they did not form alone.

Each galaxy you see today was shaped by something you cannot see at all.

Dark matter — invisible and silent — outweighs the ordinary matter in the universe by more than five to one. It does not shine, reflect, or absorb light. But it has mass, and therefore, it has gravity. Long before stars formed, dark matter gathered into halos — vast, spherical clouds — and it was these halos that pulled gas into their centers, creating the conditions for galaxies to be born.

But not every clump becomes a galaxy. Not every seed grows.

Some clumps are dark halos — massive structures made mostly of dark matter, too small or too barren to ever form stars. They remain invisible, undetectable except by the way they tug on other things.

Others are gas clouds in the intergalactic medium — relics of the early universe or the aftermath of galactic collisions. These clouds are made primarily of hydrogen and helium, the first elements forged after the Big Bang. Some stretch across hundreds of thousands of light-years, forming vast filaments that span the cosmic web. Though cold and diffuse, they hold much of the universe’s normal matter — fuel that never lit a flame.

There are stripped fragments, torn from galaxies by gravitational encounters — drifting star clusters and wisps of gas, forever separated from their original homes.

And some are failed galaxies, clumps that began to grow but lost their gas to heat, to wind, to time. Their fires never lit.

These are the ghosts of what might have been. They remind us that creation is not guaranteed. It must survive the dark to become visible.

The shape a galaxy takes — whether a majestic spiral, a smooth elliptical, or an irregular cluster — depends on how it formed and what it collided with.

Spiral galaxies are disks with arms of stars, gas, and dust winding outward like cosmic whirlpools. They are rich in cold gas, which fuels new star formation. Spirals often emerge when galaxies form slowly and relatively undisturbed.

Elliptical galaxies are smoother, rounder, and older. They are formed by collisions and mergers, where the structure of spirals is destroyed and replaced by a puffed-up, random cloud of stars. These galaxies are quieter — most of their gas is gone, and few new stars are born.

Irregular galaxies are the misfits — galaxies torn and shaped by chaotic interactions, stripped of symmetry but often rich in star-forming material. They are reminders that the cosmos is not a machine, but a story still being written.

Through it all, dark matter holds the frame. Like bones beneath skin, it determines the scale and reach of every galaxy. You cannot see it — but without it, galaxies would never have formed. The universe would be thin, scattered, and still.

The structure you belong to, the Milky Way, is still dancing through this darkness. It spins in a dark matter halo. It merges with smaller galaxies. It changes shape with time.

But nothing lasts forever.

One day, it will collide with Andromeda — another spiral in motion — and be born anew again.

Pathfinder

Galaxy formation and evolution – Wikipedia
Dark matter – Wikipedia