But the answer to "what space is" is a model of the behavior of space and stuff in it.
There is nothing "root cause" about it. It is just another part of the big picture.
It is just a more detailed model.
What is space?
How can it be bent?
Huh? That is questions we build models of space to explain.
The question we want to solve is "how does distance behave".
Everything we know is descriptions of behavior.