I don't understand physics to any degree, but I was under the assumption that EM fields also warped space (albeit orders of magnitude less). I found some explanations that are way above my head on Quora:

[1] http://www.quora.com/Physics/Can-a-large-energy-field-other-...

[2] http://www.quora.com/Physics/Can-a-large-energy-field-other-...

The second seems to suggest that energy fields, in general, do warp space.