Negative energy
From Memory Alpha, the free Star Trek reference.
Negative energy is an exotic type of energy. Negative and positive energy annihilate each other. (TOS: "By Any Other Name")
The galactic barrier has exotic properties, including negative energy readings. (TOS: "Where No Man Has Gone Before", "By Any Other Name")
The space amoeba encountered by the Enterprise in 2268 was surrounded by a layer of negative energy, which proved to be toxic to humanoid life. (TOS: "The Immunity Syndrome")
