That was an alternate dimieminson does not count according to what I read the Celestials are Eternity offspring and they are above Galactus. I read also were they said eventually he will become a Celestial they also said they same thing about Franklin Richards. Exitar is as of now and maybe always was and is the most powerful he took out the most powerful Watcher who was above the regular Watchers. I got the book where it said they givien the power to make diseisons like that, Also heard that Ego would is matched equally to Galactus in power was held from confronting another Cosmic being by the Celestials why they destroyed the being. Ego has defeated Galactus I believe and the Celestials would beat him to...
If you've read Fantastic Four#319, you know that the Celestials have a homeworld. It stand to reason then, that they have some sort of culture, civilization, and possibly, a good coffee shop, right? Wrong. The "homeworld" seen in that story was a trick, as the Beyonder later learnt. Trust no one.
Nonetheless, we DO know how Celestials are born. The Black Galaxy, a galaxy whose stars were all snuffed out by Ego, was transformed by the Celestials into a new Celestial, with the help of Hercules & Eric Masterson's DNA, and a big burst of energy from Stellaris (who was actually trying to kill the Celestials).
Given the above, it is interesting to note that the Black Galaxy, at the time of its transformation into a Celestial, was a dead galaxy. This suggests that dead galaxies are re-made into Celestials, or perhaps the "unworthy" ones, at least. We also know that Galactus is the Devourer of] Worlds, and that his destruction of life has a purpose. Could Galactus' constant ravagings have been making room for the creation of new Celestials? Food for thought.
Adding credance to the theory of Kubik and Kosmos, Eternity once mentioned how his desire to manipulate the events leading to the 2nd Kree-Skrull War led to the manifestation of the Celestials. But if the Celestials are born of Eternity, what of the Black Galaxy "birth" scene? The more we learn, the less we know.