What is the nature of "Negative Energy"

Factual Questions
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Over in the thread Big Bang Question Ring posted this link that talks about the net value of the universe equaling zero. In essence, all the positive energy in our universe (matter and whatnot) are exactly countered by “negative energy” of gravity such that taken altogether equals zero (or near enough as makes no odds).

That said it all looks to me like a vector issue and not actual negative energy.

Say you have two guys pushing with equal force on opposite sides of a cart. The cart moves not at all as the net force is zero. Each guy has a positive force acting on a vector exactly cancelled out by the other guy’s force pushing on the opposite vector.

While mathematically this may be represented as a positive and negative value which guy you give the negative value to is arbitrary. Each force is positive energy…just in different directions. Is it right then to say that one guy actually is producing “negative” energy?

Now take this idea to the link talking about negative energy. Is it really negative? Can we bottle it and use it (I am guessing no on this…at least for the foreseeable future)? What would happen to the bottle (just asking for fun…antimatter would make the bottle go boom…would negative matter/energy just make it wink out of existence)? [sub]That last is just for fun speculation.[/sub]

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One other thing…

How can gravity account for the negative energy force in the universe sufficient to cancel out all the energy present in the universe?

I thought gravity is strictly a result of the presence of matter/energy and I thought all the other forces FAR outstrip gravity in strength. Take a particle and anti-particle and let them annihilate each other and gravity will have very little to say in keeping the result pulled together. So wherever there is gravity there is matter and the matter itself comprises a far greater energy store than the gravity it produces.

Or so I understand it from my admittedly layman’s position.