"Every action have an equal and opposite reaction"
In order to save Kairi, Sora had expelled his and her hearts with the use of the Unlocker Keyblade, a keyblade created by Xehanort's Heartless named Ansem in pure darkness. In reaction, the keyblade darkness had altered Sora into a heartless. Although moment was short, it was long enough to create a nobody named Roxas in the Realm of Nothingness, but right before them outside of the Realm of Darkness is a pure heartless. At that moment, a heartless named Aros was born, but the heartless was never destroyed. Kairi had used her light to restore Sora back into his body. That kept the dark fragment in existence in his heart.
Within the strongest heart, there was a dark corridor. Surely Sora would remember the moment he was heartless, but after the rearrangement of memories caused by Namine, Aros had relocated himself in the depths of the unconscious to not be detected and live like a shadow in Sora's mind. Similar to the fragment of Xehanort's Heartless
to Riku, he rests within Sora and can always return into full existence. The same had occured with Maleficent; by a simple memory of the Three Good Fairies upon a fragment, her robes, she was restored.
The only moments Aros had made himself prominent in existence are the mere moments when Sora had seeped into an Anti-Form.
Many will speculate that if the fragment wasn't a heartless afterall. If so, then why doesn't Aros wield the Kingdom Key. It is a simple explanation in balance. The Kingdom Key is a keyblade of light and refuse to be wielded by a spawn of pure darkness. It would be very possible for Aros to wield Keyblades, but Keyblades born of the Realm of Darkness similar to Mickey's Kingdom Key Blade.
If he wished to Aros could possible create his own keyblade forge by his own darkness similar to how the Soul Eater was created, but the keyblade would lack a keychain.
"In a chemical reaction, matter is neither created nor destroyed"
Unfortunately for a single keywielder, two forms of darkness were created, the fragment that lies within him (Aros) and another.
In Neverland, Sora had met his own negative duplicate of darkness created by Riku's manipulation of darkness. This was momentarily called, Anti-Sora. While Sora was a leader of light, this inverted reflection was not.
It seems that this particular creature stands in a fine line of reality or fantasy. Was it truly a heartless or a reflection? It was a mirroring double with its own keyblade just like Sora, but being created from darkness and being a complete opposite, it lacked a heart.
Strangely it took on characteristics of a pureblood heartless. Like the majority, it had a black body of darkness with illuminating blank yellow eyes. Like a pure heartless made from a strong heart, it kept a human form.
The question now is "Was it truly an inverted reflection?" or "Is it the reflection of the darkness within Sora that had created Aros?" This could very well be a post-Aros, a heartless reflection made before the actual birth of Sora's heartless.
No matter the true answer, this other fragment still exist. It was never clear that Sora had completely destroyed the portion of himself as it returns back into the Realm of Darkness where it stayed.
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The two can exist, but the Anti-Sora wouldn't produce a nobody for it wasn't born of a corrupted heart. With the additional method of creating a heartless through a negative reflections, the worlds can truly be more endangered.
(=o please state your opinions on my mind throw-up)
to Riku, he rests within Sora and can always return into full existence. The same had occured with Maleficent; by a simple memory of the Three Good Fairies upon a fragment, her robes, she was restored. 
