Galactus, a.k.a, Galan, is a god-like supervillain in the Marvel Universe. He made his first comic book appearance in Fantastic Four #48 (March 1966). He is from the planet Taa, a world that existed in the universe before the Big Bang. When impending cataclysm starts to kill all life in his universe, he and other survivors leave Taa, but become caught in the Big Crunch (the universe collapsed and the Big Bang happened, creating the universe that we currently exist in). But Galan doesn't die from this like the other survivors, instead, he transformed through bonding with the Sentience of the Universe. In the new universe (ours), he gestates for billions of years, becoming Galactus. A Watcher called Ecce observes the birth of Galactus. He realizes his destructive nature, but chooses not to kill him.
Once he is fully aware, Galactus experiences a hunger so extreme that he consumes the life energy of the nearby planet Archeopia. This became the first of many planets that Galactus has consumed, given that he "eats" planets for sustenance. He continues this, but eons ago, Galactus realizes it would be more efficient to scout heralds to search the universe to find planets for him to consume.