That the universe began as an explosion of energy from an unknown state into an unknown state defines it as a finite phenomenon.
How did you manage to pack so much wrongness into so few words? The universe didn't begin with any sort of explosion. It couldn't have been 'of energy' even if it did. Energy isn't a thing itself, it is a
property or other things. The island of sense is that the earliest moments after what would be the singularity if we ignore everything else and just use GR, are indeed unknown. Then back to the nonsense - it didn't need to expand into anything at all (except the future), so there is no second unknown state.
ETA: Oh, I missed out the last bit of wrongness: there is absolutely
no reason to think that it is
necessarily finite.
Geometry is not going to negate nor resolve that.
Geometry is the only clue we have as to whether space is finite or infinite. It appears to be flat, and the simplest
topology associated with flat geometry is infinite.
In this instance, "nothing" is something beyond and apart from it. In the same way that "there" is "not here".
Gibberish. "Here" and "there" both require space. You can't have anything 'beyond' space, it
literally makes no sense.
Space-time has nothing to do with what is beyond and apart from space time.
There is no evidence and no scientific or logical reason to suppose that there is anything apart from the space-time.
We can't observe an "edge" that does not exist. In this instance, the "edge" is between what we know to exist, and what we don't.
The only 'edge' we have any evidence for is the limit of our observations.