I am an astrophysicist, so let me try and explain in a little bit more detail why this result is so interesting.
First of all: No, the discovery of these black holes has nothing to do with questions concerning the dark energy or missing mass. Note that one has to distinguish between dark energy or missing mass. What is meant by missing mass is the fact that in order to explain the rotation of many galaxies we need to invoke about 10 times more mass than what is found from observing the galaxies. What we do here is that we look at the rotation of the galaxies from which it is possible to infer their mass using simple dynamics arguments. In order to infer the mass present in a galaxy independently of the dynamics, you can simply make a picture of it. Since we know that typical stars have about the luminosity of the Sun it is then possible to calculate from the observed light how much radiating matter is present in the galaxy. It turns out that to explain the observed motions, about ten times more mass is required. Similar arguments also apply to galaxy clusters. This is what's called the missing mass.
Dark energy, on the other hand, is a term proposed in the Einstein field equations, and therefore also in the Friedmann-Equations, which describe the expansion of the universe. With a so-called cosmological constant, these equations predict an accelerated expansion of the universe. It turns out that this is what's observed. About 85 percent of what is causing the curvature of the universe (the so-called Omega-parameter) is due to this cosmological constant, and many astronomers call the cosmological constant "dark energy".
There is a
nice plot by Mike Turner summarizing the different terms that need to be added to explain by the observed matter density of the universe.
To turn to the question as to why we astronomers were looking for black holes enshrouded in gas: there is a long standing question about the number density of black holes in the universe. We know that in the local universe most galaxies, including our milky way, harbor a supermassive black hole in their center. These black holes are difficult to find since most are just sitting there, doing nothing. The mass of such a black hole is on the order of one million to one billion solar masses. This sounds a lot, but is really not very much: the typical radiating mass of a galaxy is 100 times more, and if you add the missing mass, then the supermassive black hole only contributes less than 0.1 per cent to the mass of the galaxy. So, on cosmological terms, the mass contained in these black holes is really negligable.
What matters, however, is that models for the evolution of black holes predict that there should be a large number of black holes that are enshrouded in rather dense material in many galaxies. It has been difficult to detect these objects so far, since the dense material absorbs most radiation from the accreting black hole. With infrared observations with Spitzer that are summarized in the press release the Slashdot posting points to it has finally been possible to confirm the long-standing assumption that these black holes exist. What is the nice thing in all of this is that these observations confirm the predicted space density of black holes inferred from previous observations, which is a very nice and important result.