quantum field theory - Virtual Gravitons?

In QED, the field strength dependence is expressed by a field of virtual photons of varying spatial density. I know that we describe gravity as a warp in space-time, but how can one warp space (and time) if there isn't anything there to warp (how does one warp a vacuum)? We have established the lack of an ether, but the Einstein tensors describe a topological warping that could be expressed as a change in density of a field. Is there any current theory involving a field of virtual gravitons to quantify the field strength?