Interpreting various equations from my limited knowledge of such a discussed area, I've come up with this:

Hawking radiation makes a black hole lose mass, hence diminishing its Schwarzchild radius. The Schwarzchild radius came up because it's a singularity (in the mathematical sense) in a solution to Einstein filed equations. The same way matter collapses onto itself up to creating such a singularity and being encompassed into the now existing event horizon, as soon as there is not enough mass to sustain the singularity, the event horizon vanishes, hence the remaining matter "pops" back into our sight.

You could view it otherwise with a thought experiment: take particles out of the black hole one by one. Each time it will reduce the mass and maybe the radius. At some point the mass/radius ratio may not be small enough to hold light and the bubble bursts in plain sight as the event horizon breaks down. In such a thought experiment, the worst case would be that the black hole would require every single particle but one to evaporate (unless you assume a single particle could be a black hole in itself).

An interesting back of the envelope calculation is computing the mass contained in a Schwartzchild radius of Planck length (the size at which quantum effects take over).

    l_P = sqrt(hG/c^3)
    r_s = 2Gm/c^2

hence m = sqrt(hc/4G) = 1.0882546265651108e-08 kg which is a bit more than 1e22 electrons sitting at a position 1e-15 smaller than a single one of them.

Is there anything to preclude there being an ultra-massive particle that might be able to cause a singularity by itself?

The particles we observe today exist in a relatively low-energy environment. Whatever's going on in the furious intensity that is the inside of a neutron star that's on the edge of becoming a black-hole could be quite spectacular in comparison.

Indeed, doesn't this violate all sorts of conservation laws when you consider hawking radiation?

We've spent decades trying to sort out the problems of black holes and information/entropy, surely to put a whole new universe inside every one is going to bugger it right up!

"And if we're in a black hole, what evidence, if any, do we have for the effects of evaporation?"

Unexplained weight loss? ;-)

I am still wondering how to square the idea that we are in a black hole with the basic principle that aside from the law of conservation of matter and energy.

I'm a complete noob when it comes to this but aren't black holes always sucking up "stuff". If we're inside a black hole does that mean that new "stuff" is added to our universe?

That's in a nutshell what I am asking.

The Law of Conservation of Matter and Energy suggests that E + mc^2 should remain constant over time. However if new stuff is being added to the universe because we are inside a black hole, E + mc^2 should be a function of time and not a constant value.

It seems there is a testable hypothesis for someone.

If it is true that stuff is sucked inside our universe from the parent universe, I imagine the same must be true for "our" black holes: they suck matter from our universe to the child universe inside them...

And having the sum of stuff sucked in and sucked out to be zero, there should be a limit to the number (or dimension) of black holes our universe can have (?)

But the mass of the black holes from the outside universe count on what I call the energy/mass balance sheet, don't they? In other words, for the parent universe I would think you get to count all mass and all energy in all child universes. But in a child universe I don't see a way to count the parent's mass and energy.

What if there's a symmetric relationship - I.e. our "parent" looks to us like just another black hole that we, not knowing better, would consider a "child" of our universe.

Hawking radiation in one is from stuff falling into the black hole from the other.

Then everyone is on everyone's balance sheet, and e+mc^2 = constant.

Topologically it would look pretty crazy.

It's said that other non-gravitationaly-locked galaxies are moving away from us at an ever increasing rate. Over time, distant galaxies will no longer be observable from our galaxy because of how fast they are moving away.

The other galaxies don't really evaporate, but from our perspective the result is the same.

Perhaps that's one possible explanation.

I've considered that connection, and given that A: it is blindingly obvious to me, a layman and B: I've never heard it from a cosmologist, I'm running on the theory there must be something very very wrong with the idea, because I'm just about 100% confident it would not have escaped their notice if it had even a faint trace of plausibility. (Second most-likely theory is they have talked about it and I've somehow not noticed, as a layman.)