User blog comment:DinoTaur/Welcome to Prof. Dino's Aurology Class!/@comment-25814498-20160218235631/@comment-25814498-20160219225347

Because of the way it reacts with physical objects around it. Energy does not have mass; that's a given, so if aura is energy it cannot have a mass either. But if that is the case, how can emitted aura react with matter as if it did have mass? When Razor through his Nen Ball at Bobobo it contorted around his face, pushed him back briefly and they proceeded to explode in a violent fashion. This would not happen if the aura didn't have mass. But, if Aura does have a mass, what state is it in? It isn't a solid as it clearly flows uninhibited during Ren for instace, it isn't a liquid for the same reason, and it isn't a gas as it doesn't expand to fill its surroundings. Therefore all we're left with is plasma. Plasma is highly charged, hot and viscous, to the level where it can actually be pushed. That's why Razors Nen Ball seemed to Contort around Bobobo's face when it made contact. That's why it pushed him back, and that's why Aura affect it's surroundings the way it does; it's matter in a plasma like state.

But notice how I said "Plasma like state". It clearly isn't just regular plasma. If it was the characters would burn themselves and everyone around them to death by simply using Ten, and unless they're running an electric current to turn the air around them into plasma, there's something else to it. Instead, I believe that like you said that I suggested, Aura is a different elementary particle. This particle can reach a state similar to plasma when given enough activation energy, but unlike plasma it requires much less energy to reach this point, and is synthesised by the user's cells using einstein's Formula of  E=MC squared. They turn the extremely high levels of chemical energy in their body into matter(Aura), which then exits their pours(Ren/Ten). As of right now I'm not sure as to how the energy is stored inside of the aura(it may connect to the frequency of the light which only Nen users can see), but the amount of energy it contains and the amount which can be produced varies depending on genetic factors and environmental ones.

This may explain the massive skill gap between the different characters in the show. Feitan, Bisky and Gon for instance may have genes which allow them to convert extremely large amounts of chemical energy into aura at a very fast rate, while a character like Zushi for instance drew the short straw, his genetics restricting the amount of aura he can synthesise. It's like body building. No matter how hard a person tries, genetics will eventually force them to come to a point where muscle mass reaches it's peak.

Humans may have evolved to use aura as a means of protecting themselves from their environment and making them able to adapt to it as they see fit. The different Nen types may have arised from different enviromental factors that the early humans had to face; enhancers for instance learning to use brute force and sturdiness to tackle their environment head on, using their aura to aid them in this, while Manipulaters for instace acted almost like parasites, manipulating wildlife around them in order to further their needs.

So in summary:


 * Aura is an elementary particle.


 * Aura is plasma but unlike most(if not all) other particles reaches this state at room temperature.


 * Aura is not stored inside the body. Instead it is synthesised from chemical energy stored inside the bodies cells(this is the reason why powerful characters like Netero and Meruem do not weigh ridiculous amounts).


 * The energy of the Aura synthesised and the amount of it depends on what genes the user inherited as well as environmental factors.
 * Because a persons genetics act as a limiting factor when it comes to aura, every Nen user will eventually reach a point where they can no longer become stronger.