The Expansion is a fictional campaign in the Marvel Cinematic Universe.
The Celestial Ego, a living planet, who could create a humanoid extension of himself, had concocted a master plan to terraform the universe's planets he had seeded into its soils. But his grand scheme was too strenuous for one Celestial to initiate alone, as they were a scarce species, so while on each of these planets, he would impregnate one native woman in hope that his offspring would carry his Celestial DNA. They had all turned out to be unsuccessful, after which he would kill and bury them in big heap hidden on his planet. When his trafficker, the Ravager Yondu Udonta, became wary to this, he decided to keep the last one he abducted, an Earth child called Peter Quill (later known to be called Star-Lord), becoming the reluctant adoptive father.
It wasn't until Quill was an adult, that Ego located him, after hearing the stories of him saving Xandar from Ronan the Accuser. Ego reached out to Quill's yearning for family after his mother died, and used that to persuade him that it was the right thing to do, but when he revealed that he implanted the terminal brain cancer into his mother (so he wouldn't be tempted to return, as he could only stay for short periods of time and for interfering with his goal), Quill came to his senses and shot at Ego. But Ego trapped Quill and drained his energy, which would eventually kill him. As the Expansion came into play and tore up Earth, the Guardians of the Galaxy saved Quill and planted bombs on the real Ego, a brain at the core of the planet, which destroyed it and everything else that he had created, including the planet, his body, and ended the Expansion once and for all.