Antimatter waste (also known as contaminated antimatter) is a byproduct of inefficient or primitive antimatter reaction techniques. As such, antimatter waste is nonreactive with normal matter and emits highly hazardous theta radiation, necessitating disposal as a toxic waste.
By the 24th century, Federation science had developed radiometric converter technology which can recycle antimatter waste, absorbing the theta radiation and converting it into energy and purifying the antimatter back into a usable reactant.
The Malon under the strong influence of the waste export industry still produce large quantities of antimatter waste, which is then transported – along with other industrial byproducts – out of their territory by Malon export vessels, to be dumped elsewhere. (VOY: "Night", "Juggernaut")