My understanding of the position of the various physicists that I follow is that they are *not* eager to assume this. It’s just, after quite a bit of investigation, the best explanation that fits the sum total of information that we have the best. I wouldn’t describe any of the cosmologists that I’ve listened to describe their position as “eager” for it to be matter. The ones I’ve listened to, in fact, would be absolutely thrilled if we had evidence that ruled out the “matter” explanation.

> Why assume that it’s matter in the first place?

I… don’t think physics assume it must be matter. It’s the leading theory largely because we’ve spent a decade ruling out various other possibilities and ideas. Dark Matter is what we have left that best fits the sum of available evidence. But, again, every physicist I’ve heard talk about this subject has acknowledged that it’s basically an area with a lot of question marks, and we don’t really know much about it, so we’ll have to keep studying the question to try and learn more.

> Because we can’t think of anything else that bend spacetime…

Again, we can think of lots of things that can bend spacetime that aren’t matter. That’s trivial. But (again, my understanding from physicists I listen to) is that we’ve thought of lots of those things and ruled many of them out (because the observation we have are more than just “bends spacetime”). Dark Matter is what’s left after ruling lots of our ideas out.

Does that mean DM is definitely the cause of the behaviors we see? No! Of course not. It could absolutely be some matter-less cause that we haven’t ruled out yet.

I would throw out their that I don’t feel like you are able to provide a summary of the Dark Matter proponents position that most Dark Matter proponents would agree with (or, at least the ones I’ve listened to; I’m sure there are some proponents who hold the positions you describe, I’m just not familiar with them).