Imagine a wide river. On a map, it's a single line, but if you look closely, there are faster and slower currents within it. The cosmic web is like that river, but made of dark matter — invisible, yet pulling on ordinary matter. Galaxies are like boats drifting along this river.
Scientists took a computer model where a billion dark matter particles moved according to the laws of gravity, and found subfilaments, smaller streams, inside the large filaments. It turns out that subfilaments are 1.5 times denser than their parent filaments, rotate twice as fast, and pull matter more strongly.
Previously, all filaments were thought to be the same, but now it's clear they are more complex. It's as if we measured the river's current at one spot and assumed it was the same everywhere. Now, to understand where galaxies are heading, we need to account for these internal streams.
The filament radius is determined by the position of the minimum radial velocity, the place where matter stops falling onto the filament and starts moving along it.