Why does dark matter form walls and filaments

I am not super well into the math of DM large scale structure, but I have a little intuitive chirp:

There is nothing in the voids pressing the DM together, it is rather the opposite: the DM is collapsing under its own gravity, which makes the DM stick together wile the rest of the Universe is expanding around it. The bubble picture is deceiving, I usually use this image instead:

It is a screenshot of a YouTube-video of a golf ball smashing a jar of Mayo, taken with a high speed camera. Here, I think the physics is more similar - the mayo is also suddenly "trying" to fill out a much larger space, while a competing force - in this case the stickiness rather than gravity - is holding it together. The result is similar, though...

I came across a very simple and elegant explanation yesterday. It combines simple conservation of energy:

$$E=W+T$$

and the virial theorem (which of course applies to the present scenario):

$$2T+W=0$$

giving the seemingly trivial result:

$$E=-T$$

So as the system moves toward a lower energy state, its kinetic energy necessarily increases, and it gets hot (so for dark matter, gains a higher velocity dispersion). It must collapse.