From what I observed on the video, there is an alternative explanation.
As a nucleus of one larger galaxy come in, about 1:04 into the video. It reminded me of a "Newton's Cradle". As the gravity of the central object is centered, the other galaxy "goes through it". Well, the momentum and inertia is greater, so it goes out farther in the beginning, and the stars get "splayed" out in a "fan-like" formation. When it comes back in, going the other way, the momentum and inertia is less, and as it goes through, gets splayed in the other direction and fans out, again and again and again, etc... THUS, you get a fanned out, "shell" effect, from the FREQUENCY of the swinging in and out of the galactic center as it is "cannibalized". The individual Stars would continue on with their momentum and inertia. As more and more nuclei are coming in, there is more and more effect. The over all impression of this, is "WAVES" of off-streaming stars. The reason why the wave pattern persists is that, as more and more stars are added to the pool, there is more mass in the outer regions and that would tend to slow down the speed and hold them out there in that pattern. So the waves do not collapse back into the center of the newly forming, larger galaxy. The mass, being distributed all round, would tend to preserve the pattern, even though Stars are continually moving back into and through the center of the forming galaxy, and they keep going out from center again, they only go as far as their momentum and inertia will allow them. As they are influenced by the warped space of other stars out there, they form a pattern. I am reminded of one of those, "tsh, tsh, tsh, Lawn sprinklers, that go around in a swath and then, "tshhhhh", back to the starting point, throwing water drops out in an arc.
So, the wave pattern we see, is from the slowing and stopping point of the momentum of the stars. There are so many doing this at the same time, it looks like a wave. But I see many waves, smaller, and smaller, and that is because with each successive pass, the momentum and speed is less and less. But there are so many, that their masses tend to hold there for a while. You can see in places where this "breaks down" and the pattern falls away.
I am sorry, but I don't see Dark Matter as influencing this. Or, not influencing it much....WHY?
Because the Dark Matter of two Galaxies in collision KEEP GOING AND PASS THROUGH ONE ANOTHER, The DM fields do not stop... even though the two galaxies have slowed, and basically have stopped. (There have been APOD pictures of the day, that illustrate this.) The DM doesn't have much, if any, affect on things. And this video shows a more "COMPACT" effect of influence. It is like two trucks carrying potatoes colliding. The two trucks smash into each other, and basically stop, but the potatoes fly on out of the trucks and keep going, and really have no influence on the crash, or what the crash looks like, or where parts of the trucks land, or in what pattern. That would be dependent on the individual parts, their masses, momentum's, and inertia, and what they HIT, and the dissipation of that force, and angle of deflection, etc... The intrinsic mass just isn't there for the potatoes to affect all that. So, while Dark Matter may have a subtle infra-structural effect behind the scenes on the clumping of matter, (and the structure of the Universe suggests this at least), it does not have the density to affect the close up movement of matter, nor what particular "pattern" such things might take.
In other words, it is all bound up in the VISIBLE MATTER, its masses, and momentum's, and inertia's, and their influences on each other. When smaller nuclei come in and go around the larger main center, they influence, and even "TAKE", stars with them. (So cool.) They leave the main center, and take stars that are further away from center, not as much influence from center, with them. That is not DM. That is not "determined" by Dm. My observation/opinion, such as it is....
It is like the end of the movie, "The Quick and the Dead". The girl shoots the bad guy in the eye, and he does a back flip. A bullet, even at that speed, does not have the mass to do that. The bullet would go through his head, and he would drop. It does not have the MASS, to lift him up off his feet into a back flip. (NICE EFFECT THOUGH). Movie Myth....Myth BUSTED!
You may not agree. That is OK.