So how do they know it is not just bent space?
Current accepted theories equate "bent space" to mass. You can't have one without the other.
If you can have "bent space" without mass then some entirely new theory about how space comes to be "bent" must be devised. A much more difficult theory to derive than speculations about the nature of this, so called, "dark matter" and "dark energy".
It is bent space. In order for space to be curved, and energy density must be responsible for that curvature. Since no other interaction can be observed in the region, the particles responsible for the energy density must be non, or very weakly interacting.