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The year 100,000,000,000,000 marked the end of the universe and end of all ages. It was further than even the Time Lords had ventured or were allowed to travel so when the Doctor's TARDIS arrived, the Tenth Doctor genuinely did not know what was outside beyond the fact the universe was collapsing.
By this time, most of the galaxies in the universe had collapsed as stars had "burnt and faded away", and most life had become extinct. Additionally, heat death occurred in this time, something that had only previously occurred trillions of years ago in 1981.
It was in this year that the last remnants of humanity flew to Utopia from the planet Malcassairo (despite hazards such as the Futurekind) thanks to the efforts of Professor Yana and the Tenth Doctor. Yana realised he was the War Master and stole the TARDIS, stranding the Doctor, Martha Jones and Jack Harkness there. (TV: ) At some point afterwards, the Doctor was able to get Jack's vortex manipulator to function and the three returned to the early 21st century. The Master, meanwhile, later returned to approximately 100,000,000,000,000 and rendezvoused with the remnants of humanity, transforming them into the Toclafane. (TV: /)
By this late stage of its existence, humanity had come "full circle" after spending aeons evolving into other forms, returning to a form similar to that of the 21st century and surrounding eras: bipedal, with traditional language skills and no apparent differences in physical makeup. The related species, Futurekind, had developed some differences but were still recognisable as humanoid. (TV: )
Orson Pink, the first human time traveller, reached this era despite heading for "next Thursday". The Twelfth Doctor found him trapped on the last planet in existence. (TV: )
When the Doctor emerged from his Confession dial on Gallifrey, (TV: ) he learned the Time Lords had moved the planet to the end of this era for its own protection to avoid their enemies. The Doctor later visited Ashildr in a ruined section of the Cloisters five minutes until the end of the universe in a reality bubble, long after where the Time Lords had hid. She watched the stars die and found it both sad and beautiful. They later left the bubble and returned to Gallifrey before this era finally ended when the bubble gave out and the "last fragment of everything there ever was" collapsed. (TV: ')