Why Is Dark Matter Essential for Understanding Gravity in the Universe?

[Chalnoth]

Very interesting discussion.

But it is something from my previous post that you don't discuss yet: "can we say that it (DM) is different by Dark Energy"?
After my post I found some strange theories based on this idea: "Dark fluid" models.

Are these models accepted by "official science"?

They're considered to be extremely speculative. Nearly all such speculative models turn out to be wrong. But I don't think they're considered ruled out just yet.

 

[Wallace]

Yes, in the LCDM model, DM and DE behave very differently. DM clusters gravitationally, and indeed this clustering is responsible for the structure we see in the Universe, but DM is also pressure-less.

DE on the other hand does not cluster (it is very close to, or exactly homogeneously distributed) and has significantly negative pressure. This negative pressure is what cause the late time acceleration of the expansion of the Universe.

Now, thinking of alternatives to LCDM there are some attempts to construct single field models in which a single energy term has strange properties that allow it to both cluster on small scales but drive acceleration on large scales. These theories are not yet fully developed (at least as I understand it) in terms of having worked out all the details and implications that such a theory would have on the things we can observe.

There is no such thing as 'official science' but these aren't crackpot theories if that's what you mean. More that they are as yet under-developed possibilities that are being investigated. I don't know a lot about this to be honest, most of what I know comes from a conversation I had for several hours with one of the proponents of this idea. It seemed there were still a lot of details to be worked out before definite model predictions could be made in order to test this against data.

Probably a case of watch this space, though my personal view is that there doesn't seem to be a great motivator for this kind of approach, apart from a desire to simplify the theory by only needing one, rather than two, new unseen forms of energy. To me it seems that the single fluid would need to in fact act in a way that's much more complex and fine tuned that the two fluids we currently put in the model. In any case it's difficult to judge this yet in the absence of more details.