The warp core is rather missnamed, it does not actualy warp space, all that it does is generate high energy plasma using a controled mater/antimatter reaction. Most of the generated plasma is sent to the warp nacels which generate the actual spatial warping. The deuturium is also used to power the fusion reactors of the impulse engines and auxilury power system. The Romulans have aparently found a way to get plasma from an artificial singularity(I would guess they siphon matter from the accretion disk or the jets perdendicular to the disk)