Negative mass/negative matter is a theoretical substance that appears in sci-fi and scientific speculation. It would be a difficult substance to deal with, if it actually exists… negative mass means negative inertia. Which means if you push on it, it pushes back, just as hard. This is why it’s considered probably necessary for stable wormholes: somehow, you line the “walls” of the wormhole with negative matter, and if the wormhole tries to collapse, the negative matter resists the collapse.
The video below explains some of the head-scratching consequences of the existence of negative matter. Not explained in the video is the idea that negative and regular matter would mutually annihilate if they came into contact. But unlike a matter/antimatter annihilation, a matter/negative matter reaction would release… nothing. Literally, nothing. One gram meets negative one gram, the end result is zero. The two substances would disappear like nothing was ever there. Of course, it’s not quite that simple. You’d probably get atoms partially torn apart… the electrons disappear but the nuclear scoots away, or the nucleus is partially eliminated, releasing a flood of suddenly liberated protons and neutrons and elementary particles. It would be nowhere near as energetic an event as a M/AM reaction, but there would seem to be a distinct sort of signature to a M/NM reaction.