Actually those three did repressent something. Galactus is the balance of death and eternity,tenebrous and aegis I believe represented darkness and sorrow and ternebrous was the caretaker of the black matter of the universe iirc. Galactus is also the previous eternity embodied basically.
Death,oblivion,infinity and eternity are the most powerful of the abstracts,she being much stronger than they all combined isn't much to say there aren't abstracts,seeing as there r rather weak abstracts around too.