Gravity. Ning never even realized that it could be one of the energies. Having forgotten nearly everything about the earth, he hadn't remembered the different types of energies as well and was waiting for the next one to be unlocked.
'I wonder if the next one I unlock is gravity or something else,' he thought.
The system actively tried to not give him information about energy or anything related to gathering a massive amount of energy in a small span of time.
So, for the past thousands of years, he had to live in the dark, unable to learn about the different types of energies.
It also didn't help that Kumia didn't really have a science-dependent society like Vilmore had come to be.
'Right, there must be quite a little information about energies in this world now,' he came to a sudden realization.
'Nice, I can finally learn about them.'