- "I call it the "Expansion". It is my purpose, and now it is yours as well. Over thousands of years I implanted thousands of extensions of myself on thousands of worlds. I need to fulfill life's one true purpose, to grow and spread, covering all that exists until everything is... me!"
- ―Ego to Star-Lord[src]
Ego's Expansion was a intergalactical conflict orchestrated by the Celestial Ego, a living planet with a humanoid extension of himself, in his quest to conquer and assimilate the universe into new extensions of his consciousness. Over the course of several millennia, Ego planted extraterrestrial seedlings on different planets throughout the cosmos and conceived several children in the hopes that one would be born with Celestial DNA.
A child from Earth, Peter Quill genetically inherited the Celestial genes and turned out as Ego's primary target. Quill was convinced to accept his father's agreement and forced to start the process of terraformation in the cosmos. This prompt forced the Guardians of the Galaxy to destroy Ego before he managed to succeed the intergalactic terraformation planning to kill all life and billions of all targeted planets' populations.
The Celestial Ego became convinced that he was the only Celestial remaining in the cosmos. Born bodyless and parentless shortly after the Universe's creation, Ego's origin remains mysterious and unknown by nature.
He came into existence millions of years ago as a flickering brain-like organism, parentless, and bodiless, floating adrift in the vacuum of space, alone.
Over this vast period of isolation, Ego learned to manipulate the molecules around himself, creating a protective sheathe. Over the course of untold eons, Ego continued to build planetary components layer by layer around this core, which eventually evolved into a life-like planet.
Ego craved a deeper meaning and purpose in life. Having wondered about the existence of other lifeforms in the universe, Ego created a biological humanoid avatar, another extension of his true self, and set out on a voyage through the cosmos. Going from planet to planet aboard his own starship, he visited thousands of worlds and numerous alien races.
Ego was not impressed by the results of his journey, deeming the other lifeforms he'd encountered "disappointing". He concluded that his true purpose in life was to remake the universe in his own image. Ego proceeded to plant seedlings of his own design on planets all throughout the universe, intending them to eventually terraform their worlds into new extensions of himself.
Ego soon realized he had a problem, however; one Celestial was not enough to activate his seedlings. He'd need an ally. Believing himself to be the last of his kind, Ego began a new campaign, siring children on thousands of female humanoid aliens. He hired Yondu Udonta and his Ravagers to track and deliver these children to him once born, though none of them proved to have inherited Celestial DNA, at which point Ego killed them, hiding their remains beneath the surface of his planet.
It was not until Peter Quill, the product of Ego's affair with a human woman, that the Celestial DNA finally became manifest in one of Ego's heirs. Quill settled on Ego's planet with the rest of the Guardians of the Galaxy, and was subsequently soon deceived by Ego in an effort to drain Quill's Celestial DNA. Ego then attempted to absorb Quill's Celestial energy into his own brain, subsequently beginning Ego's terraforming expansion in the process.
The Guardians of the Galaxy fought against Ego to prevent Ego from completing his expansion within the universe. To kill Ego, and considering that there were no other options to kill him, the Guardians of the Galaxy had to travel further underground to find Ego's brain and destroy it. While Ego fought Peter Quill, who had at that stage reached a matching god-like status, Groot planted a nuclear bomb on the surface of the brain.
Meanwhile, with every chance Ego got to absorb Quill's Celestial power, the planted seedlings expanded dramatically, consuming entire towns and all their constituent lifeforms in the process. Yet the expansion was short-lasting, as Peter Quill stalled his father enough so that the bomb which was placed on Ego's brain detonated, thus killing Ego. As a result, all of the expanding blobs within the cosmos itself ceased when Ego died.
When the bomb exploded and Ego died, the planet soon collapsed. Yondu Udonta gave up his spacesuit to Peter Quill in order to save him, while sacrificing himself in the process as he dies within the oxygen-less atmosphere of space. Nebula reconciles with Gamora, but chooses to leave anyway, and the two part as caring sisters. The Guardians give Yondu a traditional Ravager funeral, and dozens of Ravagers, including Stakar Ogord, arrive to pay their respects, releasing a string of fireworks.
- In the comics, Ego attempted to assimilate and make himself part of the planet Earth in various occasions.