Anti-life matter was a form of matter produced by the space amoeba which was harmful to life.
In 2268, Spock's telemetry information of the space amoeba indicated over forty chromosomes in the nucleus were ready to come together and reproduce. According to Captain James T. Kirk, if the energy of that organism only doubled, they'd be dead and everything within a light year would be defenseless. Doctor Leonard McCoy suggested that soon there would be more space amoebas, and the entire anti-life matter output of that thing could someday encompass the entire Milky Way Galaxy. (TOS: "The Immunity Syndrome")