- Water is analogous to the space in real world.
- Bubbles of water vapor analogous to the matter in the real world.
Now let's talk about the real world. Same as the above example, the space is intangible to us, so when mass converts into space; we only see the energy getting released and thus we say that mass is convertible into energy.
Currently this is only my hypothesis, I want to research on this, if I could get a chance.