Tens of millions of years ago, Earth was a four-dimensional space. The dinosaurs were a lie concocted to cover up the truth about prehistoric civilization.
A cosmic war once tore apart the four-dimensional light cone, and Earth fell into three dimensions. Some of the four-dimensional civilization remained in this universe. Having lost their home, they wandered the depths of the Orion Arm for billions of years, becoming known as the Wanderer Civilization.
The rest of the four-dimensional civilization was swept into another parallel light cone. They continued to exist in a detached dimension, never fully descending to a lower dimension. They preserved their four-dimensional forms and technology intact. When Zhang Yu tore open a wormhole near the Sun at the cost of his life, the two light cones connected once more. That complete four-dimensional Earth civilization crossed through the wormhole and arrived under the name of the Return.
The Wanderer Civilization stood at the boundary between two universes. They and the Returners were two branches of the same civilization: one had stayed behind and been reduced to a lower dimension; the other had fled into a parallel universe, preserving itself intact.
They did not choose humanity's side. Nor did they choose the Returners' side.
They simply kept wandering.
All they left humanity was a star map.
The sci-fi feel was pretty good, but it was too short.