The background radiation told us that on the large scale the universe is the same everywhere. Are the galaxies, then, spread evenly too?
Measure not only the direction of each galaxy but its distance, and a map can be drawn. Set it beside a picture with the same number of points scattered evenly, and the two plainly differ.
The real galaxies fall into denser and thinner places. There are wide regions with nothing in them, and galaxies crowded along their edges. An evenly scattered picture never separates like that.
Evenness comes in only on a much larger scale. Take a region bigger than a few hundred million light years and the averages agree. Within that, there is definite structure.
Looking over the map, kinds of place can be told apart.
First, there are places where hundreds or thousands of galaxies are gathered. These are clusters, the densest knots of the structure.
Knot is joined to knot by threads of galaxies strung out thinly. These are called filaments. On the map they look like lines run from one knot to the next.
Between them are left wide regions almost empty of galaxies, called voids. Counted by volume, most of the universe is void. Having anything in it is the exception.
Where did this structure come from? The seed is the blotchiness left in the background radiation, a of about .
A denser patch pulls harder than its surroundings, so it draws matter in. Having drawn it in, it is denser still, and pulls harder still.
A thinner patch does the reverse. It only ever loses matter, and thins further. Left alone, the difference widens.
A difference of became the structure we see, over billion years. In the figure the points begin scattered evenly and are then drawn toward the denser side.
Because the growing takes time, the order of events matters. Until the clearing, ordinary matter was mixed in with the light. The pressure of the light pushed back, so it could not gather however much it tried to.
Dark matter has nothing to do with light and was never held up that way. It began gathering long before the clearing, and had the frame built first.
After the clearing, ordinary matter was released from the light. It fell into the dense places dark matter had already made, and cooled there into stars and galaxies.
So the arrangement of galaxies we see traces a frame of dark matter. Only the part that shines is visible; what holds it up is not. The unseen matter the rotation curves demanded is doing the same work here.