Negative Mass-energy

Mathematically, negative mass-energy is nothing more than normal mass-energy except with a negative sign. But that negative sign makes all the difference.

Because of the difference in sign, it is found that negative mass-energy repels other negative mass-energy. But negative mass-energy is attracted to positive mass-energy, while the positive mass-energy is repelled.

This was all theoretical until the invention of the Wormhole Generator by Elias Mach in the 24th Century. The generator allowed the production of almost an unlimited amount of negative mass-energy. 

Later, it was discovered that a slightly modified wormhole generator could produce negative energy bubbles whose density would hold the bubble together until it slowly dissipated over time. This phenomenon, which is still being investigated, has already been used in unique ways, including …

Source:

Sci-pedia - The Online Reference for Science - Negative Mass-energy