Relax, tiger. The person who started the thread inquired about "the notion of eternity" and "infinite time." This is a physics forum, not a Catholic seminary.
Most physicists would probably say your question is too philosophical to merit a serious scientific response (the proverbial "what exactly do you mean?" right). However, even after all the chemical and physical reactions have taken place and there is nothing left in this universe but radiation...
Energy is the ability to do work. Work is the application of a force (push or pull) over a certain distance, so an equivalent definition for energy would say that energy is the ability to apply a force over a distance. Energy is primarily measured in joules, which is kilograms times (meters...