I don't want you to think we're ignoring your question; I suspect the reason no-one has answered so far is that your question isn't as clear as it could be. I'll try to answer what I think you're asking, but please ask for more details if my answer here doesn't help.
I think the problem is that the word "energy" has lots of different meanings and it's very easy to mix them up and end up comparing two different things. For example "energy" could mean the energy of a moving bullet or it could mean the energy in a compressed gas cylinder, and these are rather different (although they're interchangable). Dark energy is more like a pressure, though actually it's a negative pressure rather than the positive pressure you get in a compressed gas cylinder. Dark energy is (probably) a property of space itself, so it isn't like the energy you get from letting off an atomic bomb. Every cubic metre of space has a certain amount of dark energy associated with it and this energy is a constant so it's always there and always the same (strictly speaking this only applies if dark energy behaves like a cosmological constant, and there are other theories like quintessence where the dark energy isn't constant).
When you're trying to solve the equations of general relativity to find out how the universe behaves you have to count the dark energy, and when you do this you end up with the result that 72% of the total mass/energy of the universe is in the form of dark energy. But that doesn't mean you could convert the dark energy into matter, or that you could convert matter to dark energy. In fact, as far as we know you can't do this.
You're quite correct that matter can be converted into energy and back, and indeed the LHC does this all the time. Actually you do this every time you light a fire or fire a gun, though in these cases the percentage of matter converted into energy is so tiny you'd struggle to measure it. We tend to lump matter and energy together and put them in the 28% of the universe that isn't dark energy.
I hope this clarifies things a bit. If you want to ask anything further please comment to this answer or post a new question.