I had to laugh, my reply was so poorly matched to your real interest! Still, it was fun to answer as if the balloons were floating in the ISS. I'm totally unqualified to talk "dark energy", but I'm ok with water balloons. I like the family discussion topic, though!
The "spring constant" of a liquid is very high. When the exterior balloon popped, the pressure suddenly changes on the liquid balloon. The liquid would expand (a tiny bit) - and that very little expansion would allow it to drop in pressure to match the environment. So for a small amount of...
If you are looking for a very basic answer, it is that the force of gravity between the Earth and the Moon is strong enough that the Moon can't "fly away" and leave the Earth, and the Moon just happens to be moving in the right direction so that it doesn't hit the Earth.
Gravity exists...