Wookieepedia offers no explanation of this. Does anyone know what the light biots used by the Yuuzhan Vong were called? It's been a while since I read the books and don't want to dig through 20 books to find the name.
Lambents, the crystalline fruit of a plant that they grew on various planets. The level of light they gave off could be controlled telepathically and Anakin Solo was even able to use one as a lightsaber crystal (in The New Jedi Order: Edge of Victory I: Conquest).
According to Wookieepedia, they also made some use of bioluminescent lichen for lighting ships and other areas but they don't seem to have been as plot-relevant as the lambents.