We don't know what drives expansion but it has never been raw inertia from a bang. That's just the derisive name for the concept that caught on and seems to work as a concept in people's heads. Of course inertia of expanding material could never conceivably allow an expanding Universe, even something as tiny as our own Galaxy, if compressed, would have a black hole event horizon of light-decades and event horizons scale in radius linear to mass. So we've always known something else was necessary to allow initial expansion, and we've never known what that was anymore than we've known where the matter came from in the first place and how it got clumped together. One popular and recurring notion is that the laws of physics break down at certain energy densities at temperatures, like gravity might shut off, of course were that so one would expect black holes to dissolve too. Of course unifying forces and quantum and gravity has been a goal for a while and the initial expansion is hardly the only problem left hanging open ended.

For instance, while black holes work just fine in General Relativity, in Quantum and models where particles, like photons and 'gravitons' mediate forces, how would a graviton leave a black hole's even horizon to exert gravity on something? They aren't supposed to be superluminal.

Anyway that's where Dark Energy comes in, and the name is misleading. Anti-Gravity would be just as apt or Expansion Force. We don't know what the hell it is or its exact function but then we didn't know jack about gravity for centuries either. There's a lot of things that while we know how they function we don't know the why. You'll often hear me sneer at Dark Energy much as I do M Theory or other String Theory variants or Many Worlds, but that has to do with their basic non-testable - currently - nature, rather then them being bad theories. We know the Universe is expanding, we do not know why. We know, and have always known, that no simple explosion of a classic nature could drive such a thing unless the original universe at the moment of ex-nihilo bang was bigger then its event horizon, which to give an idea would be many millions of light years in radius even for lowest mass figures and just doesn't work. So a classic or even relativistic scenario under known physics can't cut it.

That in and of itself is okay, as said we still have no clue where all that matter and energy came from originally. There's lots of decent theories out there but most revolve around the idea that space itself simply flops into existence over time. Now that's a queer idea but not that queer. Space is not infinitely divisible but rather made of finite volumes, planck space. Now, I can offer an analogy that would fit an expanding universe, an accelerating expanding universe, though let me add its just a conceptual analogy not one of the specific theories.

Let's say there is a minimum unit of space, a 'space cell'. These cells themselves would be far smaller than even an atomic nuclei but no matter. I've got a big beach ball full of them. Let's say every minute there was a 50/50 chance one of these 'cells' would divide and become two space cells. So in that first minute the volume ought to go up 50%. Now, in the second minute it wants to go up another 50% but against the new volume of 150%, so at minute 2 it is at 225% of volume, at minute 3 it is at 338% then 506% at 4 minutes and so on. So if it was a a cubic foot originally at minute 1 it's .5 new feet a minute and at minute 4 it's 1.68 feet a minute. Now, theoretically a volume of space itself has no energy requirement to make, classically speaking, and the individual bits are so small and so slow to form that locally it is irrelevant, everything connects back together very nearly like nothing new was even made, but at the intergalactic scale the effect is too much distance for gravity, let alone the other forces, to tug things back together. However, we not only wouldn't know why it does this but we're not classical, space is not free but has an energy cost, and space isn't independent but is space-time, and that confuses things.

Now that all assumes a steady rate per unit volume, that X number of space cells per cubic foot are made in Y time. It doesn't necessarily need to be that way. For instance, if we pictured big beach balls filling with air from an unknown source then dividing, then everything from ambient pressure pushing on it and slowing it down to ambient temperature making the exterior plastic membrane more flexible and easier to divide could play a role. So this expansion may have happened very fast early on, with nothing interfered with it, then been dampened out by other existing forces and factors, then had that dampening effect decreased.

But it's much like Olber's Paradox, pre-red shift, we know it outright violates the known physics yet those function excellently well, and Olber's turns out not to be a Paradox at all.