The Universal Energy.

"I find it hard to believe that something that can make you thousands of times stronger than before when you don't even know what it is will turn out to be useless." 

Vyriana was positive. 

Her instincts told her the moment Nux understood what this black hole inside him was would be the moment he would become a truly broken existence that no one else would be able to fathom.

And Nux wasn't any different, despite not knowing what it was, he was certain that this black hole was what would write his future path. 

The path to the peak, the path to become the strongest. 

He just needed to understand what this thing was. 

And that's what he had been trying to do, when others trained, he used the chance to enter the black space again, he didn't understand anything, but he believed just staying here made him closer to it and since following his instincts was the only thing he could do right now, that is what he did.