The galactic barrier, a form of negative energy
Negative energy is an exotic force, whose relationship to normal energy is analogous to the relationship between matter and antimatter.
The barrier at the edge of the galaxy is composed of negative energy. In 2268, when the Kelvans took over the starship USS Enterprise and set a course through the barrier, Spock and Scotty devised a plan whereby they would flood the warp nacelles with positive energy, which would have led to the destruction of the ship, although they were overridden by Captain Kirk. (TOS: "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")