This might help explain it:
QUOTE
Dr Michio Kaku: The amount of energy necessary to create a bubbled universe is zero. Matter has positive energy, but gravity has negative energy. The sum of the two might be zero. This means that bubbled universes can be created all the time. So the Universe is for free.
So maybe it's balanced by all the matter in our universe that's leaking out through black holes!
I also found the answer to my question:
QUOTE
We believe that a multiverse of universes exist like bubbles floating in Nothing. Each bubble forms as a quantum fluctuation in Nothing. We feel that as this bubble forms its matter is dominated by strings and membranes which create musical notes which we see as particles of the universe.
Boiling water is a purely quantum mechanical event. If water molecules were like billiard balls, water would never boil. Water boils and a tiny fluctuation occurs in water. Similarly, universes may be created all the time, even as we speak, due to fluctuation in Nothing. In other words, universes may be for free and Big Bangs happen all the time like in boiling water.
The transcript of the programme is
here, and the transcript of an online chat about the programme is
here.