This leads to an interesting question the article didn't mention...
If a galaxy can exist without any dark matter and others with a balance of dark and normal matter, can one exist made entirely of dark matter?
If a galaxy can exist without any dark matter and others with a balance of dark and normal matter, can one exist made entirely of dark matter?
But I don't think we know of a mechanism for such a structure to form naturally. Normal particles collide and transform some of their gravitational energy into heat. If dark matter only interacts gravitationally, blobs of it would pass right through each other without losing energy. I think(?) you need these sorts of collisional energy losses to get matter to coalesce into a galaxy.
The formation of a dark matter galaxy in such a fashion is plausible, but would take a long time.
I just found the video! ("Gravothermal Catastrophe")
One of the problems with the current best model of the universe (Lambda CDM - a universe that is primarily make up of some cosmological constant driving expansion and cold dark matter whose gravity drives structure formation) is that it predicts that there should be far more of these dark matter overdensities than we see galaxies. This is the "Too big to fail" problem - https://arxiv.org/abs/1103.0007 is a pretty readable description. Basically the model predicts lots of dark matter clumps orbiting the milky way and we don't see enough satellite galaxies to populate them.
So there are now two options: 1) Galaxy formation becomes stochastic at lower dark matter halo masses - and so there are lots of these galaxyless overdensities of dark matter. 2) There is something wrong with our model and there might not be these overdensities. 3) The milky way is an outlier.
I think the short summary is that, at very low masses (say ~1e6 solar masses) we wouldn't be surprised if not all dark matter halos form galaxies. At high masses (1e12 solar masses, roughly that of the milky way halo) we would be very surprised if there weren't galaxies. But in between we are not sure.
Edit: To be clear, these structures would still likely contain baryons (normal matter), they just haven't formed stars (or would have formed only a very few stars).
So in that scenario it's quite possible that the dark matter part of those galaxies is drifting around on it's own somewhere. I don't know that we'll have any way of detecting them soon though...
If dark matter is theorized to explain more gravity than there would be without it, that implies dark matter has a gravitational effect, which means it too might coalesce into very dense things. Perhaps dark matter also forms star-like gravitational bodies, but whatever makes it "dark" hides those too.
It seems weird because I'm almost suggesting... Cloaked stars. Which on the face seems terribly unscientific, given what we know about EM radiation of things. But i think dark matter implies things that don't radiate in the EM spectrum.
Part of me wants to speculate that dark matter is a gravitational force echoing from 4th dimensional matter. Like if a cube affected a plane over which it hovered. But that's just me being an ignorant speculator.
Each cloud just continues with the same speed in the direction it had. They dont interact other than gtavitationally.
After passing each other they start to pull on each other gravitationally and slow each other down. Eventually stop and accelerate towards each other again.
And so they can oscillate forever back and forth, if left to themselves without other clumps. But at no point would they "clump"... they are only close to one another in space when their speeds are most opposite one another. Being clumped means being im the same location and having the same speed.