just a remark to the last paragraph:
"Of course, if "energy cannot be created", then we're left to wonder how, according to this subtrope, "Pure Energy" came into being in the first place."
the "obvious" answer would be the same way as in our world. we have lots of energy too and it cant be created (well it can, but not for long).
The Cosmopolitan Fictioneer
According to this part from the Other Wiki's article on matter
, matter can be composed of massless particles such as photons, or even from "pure energy". No, I only wish I was making this up; that part even has citations.
this pure energy isnt the same as in the trope. there is even an example for "pure energy": kinetic energy. the reason for what you quoted is quite simple: mass equals energy, the gravitational force depends on energy. a complet explanation would be quite long, and i would suggest some books about general relativity and particle physics
Isn't 'Pure Energy' just assumed to be physical phenomenon that remains stable at state between matter and energy. As if, when matter comes into contact with anti-matter, the two 'annihilate'. Forgive me if I'm wrong, but I always assumed that meant they turned into the raw energy used in those massive explosions they liked to talk about on the discovery channel.
It seems like the Authores are treating it as matter that has been mostly 'unraveled' that bears the best of both matter an energy, being tangibile enough to kick ass but intangible enough to float into whatever its powering without any visible inlet. Of course, I'm assuming that all of this would be turned into self-containing shapes by Rule of Cool.