What is the thermal energy?

Perhaps the heating power all over the universe.
Heating means accelerating of the speed of molecules' entropical movement. So, the fewer molecules, the cooler atmosphere.
The principle of the entropy increase holds good among the rigid bodies only.
(Copier here is just a volunteer help, not a thief, to the last. He does not steal anything. No name, no salary for ever. But even a reporter help himself has to know things to be a reporter.)