[ExGeeEye]

What would you call the place where the marble-sized universe was located? What were its physical properties? Do we say that an infinitely large space containing nothing, a total vacuum, was then and has since been filled by a universe of objects ranging from galaxies to sub-atomic particles?

Would you want to stand next to the marble-sized universe, if you could be assured of escaping to a safe distance before detonation?

[snarks_when_bored]

Good questions. I think we're still trying to find the answers.

But as for your last question, there's no escaping the birth expansion of a cosmos! If you're anywhere nearby when it starts, you'll be gone in a trillionth of a second or less.

[AntiGuv]

String theory would seem to suggest that our universe can be imagined as one of many, like bubbles in a froth, and as each expands (or contracts) it 'merely' inflates or conflates the froth. In that manner of envisioning the universe, there's nothing 'next' to the universe within our timespace; you cannot travel to the 'edge' of the universe because there is no edge, except as a purely abstract mental construct.

[Doctor Stochastic]

What would you call the place where the marble-sized universe was located?
Everywhere.

What were its physical properties?
Very dense.

Do we say that an infinitely large space containing nothing, a total vacuum, was then and has since been filled by a universe of objects ranging from galaxies to sub-atomic particles?
No. Space is what is expanding.

[RightWhale]

Do we say that an infinitely large space containing nothing, a total vacuum, was then and has since been filled

That is not in any of the major mathematical models. What is in the models is that the universe, when it was microscopic and now when it is too big to imagine very well, was complete in itself: no outside to expand into.

[BeHoldAPaleHorse]

Would you want to stand next to the marble-sized universe, if you could be assured of escaping to a safe distance before detonation?

Stand WAY back, there, sonny boy!

The last thing heard before the Big Bang: "Hold muh beer and watch this!"