Hawking and others have claimed that there's a negative energy associated with gravitation that makes the universe's total energy balance -- including all the energy in all the mass in all the matter in the universe -- zero. But there was no such thing as negative energy in any type of physics that I've studied, so my questions are: What are examples of negative energy? What's the evidence that it exists? What does it do? And how much of it is there? There's supposedly a universe's worth of it around, but I don't know of any. A friend thought Hawking was talking about gravitational potential energy, but that's just a form of regular positive energy. My only idea was a motion-slowing negative energy effect from the time-slowing caused by gravity in general relativity, but that's probably tiny compared to the energy in the gravitating masses.