Photonic matter is an energetic substance which can be found in protostars.
In 2371, upon detecting unusually intense photonic activity in a protostar in the Delta Quadrant, the crew of the USS Voyager beamed aboard samples, hoping that it could boost the efficiency of their power converters. It was soon discovered, however, that they had, in fact, abducted photonic lifeforms from their natural habitat, and the lifeforms began to retaliate by abducting members of the crew and converting them into photonic energy. Upon releasing the lifeforms, the Voyager crewmembers were returned. (VOY: "Heroes and Demons")