Xeelee

The Xeelee /ˈziːliː/ are a fictional hyperadvanced species from Stephen Baxter's Xeelee Sequence. They were first remotely mentioned in the 1994 novel Timelike Infinity and were later central actors of several novels and a substantial number of short stories. Within the Xeelee Sequence, they are considered to be the most advanced of all Baryonic life-forms and possess technology and abilities far beyond any other spacefaring civilization. They have demonstrated the ability to routinely construct Closed Timelike Curves and have exploited time travel to engineer their own evolution and history as far back as 13.5 billion years ago (within two hundred million years of the Big Bang). Their abilities in the series far exceed that of a Type III Civilization on the Kardashev scale.

Although the Xeelee seldom directly appear or take action in the stories their impact on the development of most civilizations and the universe as a whole is profound. The Xeelee frequently leave behind derelict ships, tools, and other materials giving the appearance of thoughtlessness or reckless abandon at what a junior species might do with it. Most junior species see no benefit in developing a technology on their own when they can simply steal it from the Xeelee debris. This has a profound influence on the various galactic economies and technological development of junior species.