Is there a cosmological constant? By which I mean an expanse of space free from matter, forces and curvature where time or space/time can flow unhindered at its natural rate.

I understand the measure of time to be observationally relative and affected by mass, motion and energy and wonder if it's possible, theoretically or otherwise, that there is a natural state of being, in terms of space, by which all else can be measured?

I suppose this question relates to 'what is space?' as it would appear, logically at least, that in order for there to be curvature of space there must exist a space to curve. Is space itself the constant and the observed phenomena of distortion of space/time merely localized events?