Actually, that first statement is not correct. The universe isn't expanding due to dark energy. It's accelerating due to dark energy. The normal expansion, called metric expansion, is an effect of general relativity. When you get a homogeneous distribution of matter or radiation(a perfect fluid, a uniform gas, radiation, a homogeneous distribution of galaxies in the case of the universe today), you can solve the Einstein field equations for general relativity for an expanding universe, called the FRW metric. In this metric, the distance in between bound objects (i.e. galaxies today) increases over time. This doesn't require dark energy, just a universe filled with matter or radiation.
A simple article about some misconceptions about the expanding universe is here, I recommend reading it:
So, prior to the discovery of dark energy, the expanding universe was understood perfectly well. However, we assumed that this expansion was slowing down. However, a discovery in the later nineties that was awarded the 2011 Nobel Prize in physics showed that not only is the universe expanding, it's accelerating. So, this is the role of dark energy.
Many different candidates for dark energy have been proposed, but one is heavily favored, the cosmological constant. In Einstein's equations for general relativity, you can throw in an extra term, $\Lambda$, the would play the role of a negative pressure vacuum energy. This has the effect of accelerating the expansion of the universe.
Why are we confident dark energy is just a cosmological constant? One of the defining features of a cosmological constant is its equation of state. The equation of state, $w$, is given by $p \over \rho$, where $p$ is the pressure it contributes, and $\rho$ is the energy density. A cosmological constant has $w=-1$. The WMAP seven year report recorded the value as $w=-1.1 ± 0.14$. Within the error margins, the cosmological constant fits very well.
Quantum field theory also predicts the existence of a vacuum energy, so it was hoped that this would match the value of the cosmological constant. However, the value calculated by QFT was enormously higher. Using the upper limit of the cosmological constant, the vacuum energy in a cubic meter of free space has been estimated to be 10^-9 Joules. However, the QFT prediction is a whopping 10^113 Joules per cubic meter. This is the 'vacuum catastrophe'.
For a simple page from the Usenet FAQ about the cosmological constant, see here:
For a very thorough description, see here:
So, because the cosmological constant works so well as a description of dark energy, and is supported by the evidence, we prefer that over a description such as quintessence, or something similar to the explanation you proposed.
Addition - Regarding the quantum fluctuations:
The very early universe was filled with an obscenely hot and dense plasma and a bath of radiation. The metric expansion of space cooled and redshifted the radiation, and broke up the plasma into a much less dense gas of hydrogen. This is the essential nature of the big bang model, which you should note has nothing to do with a 'bang'. The model was been confirmed by observations, which you can read about here.
However, there are a few problems - first is the flatness problem. We observe that the universe is very, very close to being spatially flat. Since expansion would cause the universe to deviate away from flatness, it must have been even flatter at the time of the big bang. Ridiculously flat. How did it get this way? Second is the horizon problem. We observe that the universe is homogeneous on large scales, that is, it's pretty much the same everywhere. This means that primordial plasma must also have been perfectly homogeneous, which is confirmed by observations of the cosmic microwave background. However, if the expansion of the universe was extremely rapid from time zero onward, how did this plasma come to equilibrium? It certainly wouldn't have the time to do this. And third is the monopole problem. Grand Unified Theories, or GUTs, are theories that unify the electroweak interaction with the strong nuclear force. They have the unfortunate feature of predicting that hot temperatures of the early universe should have produced an abundance of heavy magnetic monopoles, which we certainly don't observe. Fourth is the homogeneity problem - why are there no inhomogeneities besides galaxies? What made the early plasma so 'smooth'?
A model called inflation fixes all of these problems. Inflation proposes that the very early universe underwent an enormous expansion, growing the universe by at least 60 $e$-folds. This expansion would be driven by the inflaton field. This field would reach an undesirable energy value, called a false vacuum. When it's in this false vacuum, it has the property that it exerts an enormous negative pressure (somewhat similar to dark energy). This drives inflation. After a very short period of time, the inflaton field reaches it's true vacuum (through normal quantum effects such as tunneling). When this happens, it decays into a bath of radiation, heating the universe so that the big bang model can go from there.
So, how does this solve the problems of the big bang model? Well, the enormous expansion would eliminate any curvature, making the universe extremely flat. This solves the flatness problem. Second, it would allow the universe to expand very slowly before inflation, allowing it to come to equilibrium. This solves the horizon problem. Any monopoles produced in the early universe would have been spread out so that we would only see about one in the entire observable universe, so the monopole problem is solved. And finally, inflation would 'iron out' any large scale inhomogeneities with the rapid expansion.
So, this is where those quantum fluctuations come in - prior to inflation some regions of the primordial plasma would have become very slightly denser due to quantum fluctuations - when the universe inflates, the random changes in density that come from quantum mechanics will get magnified, and you end up with what's called a "scale free power spectrum."
It's like drawing a small line on a flat balloon. Blow the balloon up, and the line will become very large. Similarly, small density perturbations become primordial 'seeds'. Since these are due to random fluctuations, we would expect this to produce a universe that has an even distribution of galaxies, such as ours. From there, dark matter clumps around these seeds, which then draws in the rest of the matter to form proto-galaxies. From there, full galaxies develop.