Negative energy is difficult to prove because, like a black hole, it can never be directly observed. However, experiments have shown as conclusively as possible that it is real. That is to say, certain QM equations predicted it, and described some of its predicted behaviors. Measurements from supercoliders match the modeled predictions of what those measurements should be if negative energy exists. http://www.compukol.com/mendel/articles/Quantum_Negative_Energy_Problem.pdf" to an article discussing it. Basically, if you take a wave that approaches zero energy at its troughs, then compress that wave so that the peaks get taller and the troughs get deeper, then the lowpoint of the wave can get below zero.
However, this means that negative energy probably can't be used for stabalizing wormholes and whatnaught. Notice that one of the rules of negative energy is that it can only exist as part of a system that contains more positive energy, so the net result will always be positive.