Matter, of course, has positive energy. But gravity has negative energy. (For example, you have to add energy to the earth in order to tear it away from the sun. One separated far from the solar system, the earth then has zero gravitational energy. But this means that the original solar system had negative energy.)
If you do the math, you find out that the sum total of matter in the universe can cancel against the sum total of negative gravitational energy, yielding a universe with zero (or close to zero) net matter/energy. So, in some sense, universes are for free. It does not take net matter and energy to create entire universes. In this way, in the bubble bath, bubbles can collide, create baby bubbles, or simple pop into existence from nothing.
http://www.kosmografias.com/blogue/?p=552
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