The Tkon Empire was a massive empire that existed over 600,000 years ago. The Empire had a population of trillions and was incredibly advanced technologically. The Tkon are rumored to have had the ability to move entire star systems, using planets as outposts for defense. Their empire collapsed not long after the Age of Makto when the central star of their home system went supernova.
The Empire employed guardians known as Portals to defend its outposts and to act as gatekeepers for those seeking entry to the Empire. At least one such guardian, Portal 63, survived and was encountered by the Federation starship USS Enterprise-D on a planet in the Delphi Ardu system in 2364. This occasion was the first instance of contact with the extinct Tkon, as well as with the Ferengi , whom the Enterprise had been pursuing under suspicion of theft.
Beyond some information regarding planets in the Empire and an understanding of their script and some of their history, little is known of the Tkon by the Federation. Even students of archeology such as Jean-Luc Picard had limited knowledge in the 24th century. (TNG: "The Last Outpost")