>>16716854 (OP)
Another universe > black hole > stuff traverses to "singularity" or Einstein Rosen bridge > into white hole (we are here)
We are inside a white hole, the singularity only exists in our past and we can only move away from it (time reversal of the inside of a black hole) therefore everything is moving away from and expanding from the singularity. Eg time has an arrow of direction - it only moves away from the singularity
The boundary of the universe is the inside of the event horizon, which stuff can cross over out of, but nothing, not even light can make it back inside > the absolute visibility limit, you cannot see past this even if you were close enough to it, though we can't catch up to it
Technically speaking the universe could gain mass this way from the event horizon, via time reversal hawking radiation. Outside the event horizon of a black hole particle pairs positive and negative form and more negative particles fall into the black hole and annihilate with positive particles, causing a black hole to evaporate over time if it doesn't take in enough mass
In a white hole it's time reversed, particle pairs form on the inside of the event horizon (reversal of combining), and more negative than positive particles cross the event horizon and exit the black hole, building up more mass as the universe expands potentially