Right, but even if you discount the mass, you’ve still got an acceleration curve, and mathematically, that doesn’t happen without some constant additive factor. Normally that is a force, but I suppose it could be something else if we aren’t talking about accelerating masses. Still seems to me that an undescribed fundamental force is the natural suspect.
I guess it just goes back to the mysterious nature of space-time. I mean, we still don't really know what causes gravity, a supposed "curvature of space-time".