That's such a mind-bending question for me, and I'm not smart enough to posit a response in terms of mathematical physics, but philosophically, I do not like to put the answer in the hands of religion, either, because you end up with circular explanations for things that defy logic. I have to just say that I don't know, but I think there's some scientific way to explain it, and no one has done that yet(to my satisfaction).

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The mind is a weird and mysterious place

Rather than bring up the 2-dimensional analogy of "expanding into" let me simply restate that there is no more space than what the universe currently occupies at any instant. If it is expanding then it creates the space as it does so. There is no more 3-dimensional space at that moment. You could postulate another dimension into which it is expanding but if you limit your imagination to just our apparent 3 dimensions then you must try to wrap your mind around the idea here and that there is nothing North of North on our globe since the Mercator system is strictly 2 dimensional.

AFAIK black holes do not explode,they rather implode. If the Hawking theory is correct, they emit radiation and shrink in a very long timescale.
Tullio

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The Universe can expand without there being anything outside it for it to expand into, says science writer and astrophysicist Adam Becker. He explains this mind-bending idea to BBC Earth's Michael Marshall and Melissa Hogenboom, with help from the animators at Pomona Pictures.
http://www.bbc.com/earth/story/20170210-how-the-universe-can-expand-if-there-is-no-extra-space

The Big Bang dot was not a super massive black hole.
Totally different objects.

Oh. They have one thing in common though.
They are very very tiny dots with no matter or particles. Only pure energy.

The black holes, already envisioned by Pierre Simon de Laplace on the basis of Newton's corpuscolar theory of light, were only a mathematical curiosity until Stephen Hawking and Roger Penrose in a seminal paper treated them as physical objects. Then came the astronomical discoveries.
Tullio

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Not only Laplace.
The amateur astronomer Reverend John Michell in 1783. Ops. A Reverend:)
Then Pierre-Simon de Laplace in 1796.
Then in 1916, the astronomer Karl Schwarzschild.
https://phys.org/news/2017-04-black-holes-theorized-18th-century.html
A bit off topic but Black Holes have some in common with Big Bang dots when it comes to the their physical properties and can shed some understanding of the origin of our Universe.

The universe is everything we can measure. The observable universe is everything we can directly measure.
This allows things to move into and out of the observable boundary but still be in the universe.
e.g. Dark Flow

Or someone in a galaxy near the edge of our observable universe has a very different observable universe than we do. He can barely see us, but looking in the opposite direction he sees 1/2 a universe that we can't, the same applies to us when we look the opposite direction from him.

Dark Flow.
I mentioned in another thread about "mother Multiverse" and the navel.
Yes. Mersini-Houghton and Kashlinsky apparently found with help of the WMAP a region where galaxy clusters seems to move from our Universe.
And then suggesting that our Universe once was connected to "mother Multiverse".
Well, this is very speculative:)
Maps of radiation left over from the Big Bang may show traces of universes besides our own.
http://discovermagazine.com/2014/oct/18-beyond-the-outer-limits

Here are 5 scientists thoughts about the origin of the universe.
https://www.closertotruth.com/series/how-did-the-universe-begin

It’s among the oldest questions because we humans are rightly obsessed by ultimate origins. Cosmologists can now explain back to the first 10^-36 second of our universe - with the theory of “cosmic inflation”, which is what put the “bang” into the Big Bang. What is recent thinking on the beginning of the universe?

Oh. Theories of the origin of the universe has even a name.
Cosmogony:)
Björk's thoughts https://www.youtube.com/watch?v=yHCQ6Pgfk28

Don't we have to decide what shape the Universe is?

https://plus.maths.org/content/shape-and-fate-universe

And was the triangle used to calculate the curvature big enough to say that our Universe is really flat?
We don't even know the size of our Universe.

With the plausible assumption that the size of the universe before the inflation occurred was approximately equal to the speed of light times its age, that would suggest that at present the entire universe's size is at least 3×10^23 times the radius of the observable universe.[25]
There are also lower estimates claiming that the entire universe is in excess of 250 times larger than the observable universe[26] and also higher estimates implying that the universe is at least 10^10^10^122 times larger than the observable universe.

Heard this quote in a TV commercial "The universe is so universal." ;)

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Bob DeWoody

My motto: Never do today what you can put off until tomorrow as it may not be required. This no longer applies in light of current events.

Exactly what I have been saying for years that our silly little LOCAL Big Bang is expensing in to the MAIN universe.

But if that is the case wouldn't some of the MAIN universe be inside our LOCAL universe. And therefore we should be able to find galaxies older than 13.8 billion years and so far we haven't.

But our local universe is a part of the main universe that actually is nothing.
Our universe expand, creating space and time, in to the main universe and only a part of it is observable due to the speed of causuality limit .
We can only observe things up to 46 billion lightyears from us.
There are most likely galaxies outside that boundery but we will never be able to observe them.
Further explainations:
What Happens At The Edge Of The Universe?
https://www.youtube.com/watch?v=AwwIFcdUFrE

How do you know there is nothing in the MAIN universe?

Who is this guy ?Get him a proper shirt.
The universe is FINITE and UNBOUNDED. It is putatively expanding but not at the speed of light. It is about 13.8 billion years old
Even though there is a somewhat lace-like structure to the distribution of Galaxy clusters, The universe is essentially homogeneous on a large scale.
If we could establish a baseline that was long enough, we would see the curvature--most likely we are a Sphere since the big bang would have expanded in all directions. Remember that the "inflation' lasted an extremely short time.

So I posit that we are basically inside a sphere (3 physical dimensions) There is no edge in 3 dimensions we would simply double back along a geodesic if we traveled in what we thought was a straight line for a good long time.
If we are expanding it is the case that gravity has not had sufficient time (or mass) to slow the expansion due to the big bang. Since we can't see far enough, and if we could, we would be looking back billions of years ago. We should be able to see this expansion by looking at nearby galaxies. However we seem to say that the farther out we look the faster the galaxies are receding--this suggests to me that we are actually contracting. Gravity does have an effect since the Andromeda Galaxy is racing towards us (and we are racing towards them of course) if this is a typical density of matter then I say we are contracting or at least gravity is slowing the expansion and it is not accelerating. Theories that space creates more space not withstanding.

I don't think I really know:)
Anyway. Outside the boundary of our universe there is nothing.
Otherwise our universe cannot expand.
But the MAIN universe is very BIG and if our LOCAL Big Bang happened here so why can't there be many more universes like ours out there.
There are even some scientists that believe our universe is or has been connected to one other universe after studying the WMAP.
This is very hypothetical of course.

How do you know where our universe formed, inside or outside the main universe.

Most likely we are the only Universe. There is no other "Where" where the other universes formed--that is all of existence.

How can we test a multi-universe theory ?

and, perhaps more surprisingly, some of the galaxies we can see right now are currently moving away from us faster than the speed of light!

If something is moving away from us faster than the speed of light, then we would not be able to see it as the light will never reach us.