Galactus is someone who can change any planetary element into energy and feeds on it. This character debuted in Fantastic 4 Issue 48, his character was named Galan and lived on the planet Taa which has been in the universe since even before the Big Bang. He is very old, in existence since even before the Big Bang. So much so that he gained his powers because of it.
At the time of the Big Bang, this character and his planet faced a terrible fate. Because of the radiations of the Bang that was about to happen, the people of his planet were dying and he wanted to save them all, because of which he took his space ship and went right into the Big Bang, where he had thought of sacrificing himself n order to save his world.
There he sees many ‘sentiments of the universe’ and instead of him dying, they all merge with him.
After the universe is formed his body is seen by ‘The Watchers’ floating in the universe in the form of pure energy. He then creates a suit for himself in order to contain his energy. He after that, in order to feed himself, goes to planets with no life, but after some time, he thought of himself as God and starting obliterating the planets even with life.
In one such event, he goes to the planet Zen La, where a person says to him that he’ll be his herald if he spares his planet. That person is then named The Silver Surfer and turned into a cosmic being.
Not just that the Surfer then finds planets for Galactus which leads them to Earth, where the Fantastic 4 defeat Galactus when ‘The Watchers’ aren’t able to. Silver Surfer also sides with them. Galactus is blackmailed by Reed Richards when he finds a gun from Galactus’ home planet Taa 2, which he recreated and renamed. Reed asks him to leave Earth or get obliterated, with ‘The Nullifier’, a weapon that he brought from Taa 2.
After this Galactus leaves but comes later to take Silver Surfer back, where he’s defeated by the Avengers.
Galactus is one of the most powerful beings in the Marvel universe and has existed since before the Big Bang alongside, the Eternity and the One Above All. This foe is yet to be seen doing justice to Galactus- the Devourer in the films. We hope that soon he’s given a grand role just like Thanos in the MCU.