It's speculation- just so you know. Not theory.

Negative Energy has been speculated in the case of like, for example, time travel.

An analogy would be:
Take "energy" and place it in a box. (Imagine a sine wave inside a square).
Now, compress the energy until the tips of the sine wave in your square protrude Outside of the lines of the square. Those tips would be "Negative Energies" that, speculatively, would exhibit strange properties.