As I understand it, dark matter is our best guess at explaining an observation of mass via gravitational lensing and other effects where there is not enough visible matter to account for it. What I never see mentioned, and perhaps there's a good reason for this unbeknownst to me, is the possibility that this phenomenon exists independent from any matter, just as we do observe it. It seems to suggest that without matter space time would be flat across the entire universe. Why isn't it possible that space time is inherently imperfect, with it's own curvatures independent of mass? I've never seen this question asked, or answered. Maybe I've been looking in the wrong places, though. Can someone enlighten me?