The Heart of the Universe was presumably created by Marvel’s Supreme Being to correct an imbalance in the fundamental architecture of the Marvel Universe. Apparently, the fundamental force of Good and Evil were out of balance with many of the heroes of good dying in the recent years and the forces of Evil growing. The force of Good seeking to right this imbalance recycled (resurrected) many good heroes that had fallen and some more than once. In doing this the more fundamental balance between life and death had been set askew, causing a cosmic cancer that was destroying the foundation of the entire Marvel Universe.
The first beings to come across The Heart of the Universe were an alien race seeking a method to bring absolute order to the universe. Calling themselves the Celestial Order they sought out beings from each sector of space to infuse with the Heart’s power so that they could enforce the Celestial Orders beliefs. The process of infusing such a being took thousands of years. The first such being to emerge from this process was the Egyptian Pharaohs Akhenaten.
In Akhenaten arrival to earth he easily destroyed Xavier’s Institute for Higher Learning and all of the Mutants there and many High Fathers and most of Earths heroes with little effort.
Eventually, Thanos manages to take The Heart of the Universe in full and easily overpowers Akhenaten.
Thanos has difficulty in assuming the power of The Heart of the Universe, but eventually does succeed. He begins to understand the nature of The Heart of the Universe, and comments that even possessing the Infinity Gauntlet pales in comparison. He described the possessing of the Infinity Gauntlet as external control where as possessing The Heart of the Universe was being the very fabric of existence in all it varied forms.
Thanos use The Heart of the Universe to undo all the damage that Akhenaten did, and by doing so gained the attention of the Living Tribunal who then gather’s all the cosmic abstracts and Marvel’s Heroes in an attempt to take The Heart of the Universe from him. Their attempt fails, but in his divine rage Thanos absorbs the entire Marvel Multiverse including even the Living Tribunal. Only those in Atleza's domain (and Lady Death) were unaffected by Thanos’ absorption, which included Atleza, Gamora and Adam Warlock.
Eventually Adam Warlock helps Thanos realize the folly to which he had been tricked and Thanos reconstitutes the Marvel Multiverse and everything in it, except The Heart of the Universe (presumably). The only being in the Marvel Multiverse that remained aware of what had transpired was Adam Warlock and the only change that was supposed to be made was that there would be no miraculous resurrections or bargaining with the great divide (i.e. Death).
in a marvel handbook it states that the heart of the universe is only a aspect of eternitys heart.