Photonic matter

From Memory Alpha, the free Star Trek reference.

Photonic matter is a photonic energy substance found within protostars.

In 2371, the USS Voyager collected photonic matter samples from a protostar in the hopes of using it to increase the efficiency of their power convertors. However, it was found that the matter was actually photonic lifeforms, whose capture caused others of their kind to begin taking Voyager crew members from the holodeck. (VOY: "Heroes and Demons")