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Warp plasma is the primary energy used to power the warp coils and thus create warp fields. These plasmas were similar in many ways to stellar plasmas.
Several times during the production of various Star Trek series there have been instances when our daring crews have needed to use warp plasma as a way of paying for supplies or as bargaining chip of some kind and our intrepid heroes seem to simply go to the nearest warp plasma faucet and fill up a bottle or other container. This isn’t possible. Plasma is a form of matter in a highly energized state, most often a super-heated state.
Thus, it is always very hot and must be kept hot or it simply returns to its cold matter state and thus likely loses all of its ‘special’ value.
(At least they haven’t tried to pour it on the floor yet!!)
The Matter-Antimatter reactor supplied the power to the warp coils, which were really just big fancy magnets. The coils manipulated the plasma, producing the 'warp field' which allowed them to slip through space so fast.