A few years back now, in my intro Cosmology course, one of the things we discussed was the topic of "dark energy", a mysterious energy which seems to be causing the universe's expansion to accelerate instead of slow down as had been previously expected.
The biggest piece of evidence in this was trying to fit observations to models of the universe in which there were different amounts of matter and energy.
Much to the surprise of researchers, the evidence favored the model which had this mysterious dark energy pushing the universe apart.
Although the data seemed to fit very well, I've never been terribly comfortable with the whole idea of dark energy. The concept of this energy being in an amount very similar to the amount of the total matter that comprises dark matter makes the whole thing rather suspect to me. But as I mentioned, the data fits well, and there's no better explanations out there, so I give the theory my support, for whatever that counts for.
However, that comes with the reservation that, as with all theories, it undergo more testing with ever increasing precision to make sure things aren't a bit wonky.
When I first learned about this theory and did my reading of the papers and other background, I learned that a probe named SNAP (SuperNova Acceleration Probe) was in the works which should dramatically increase the precision of the research.
But aside from that initial report of it, this probe dropped off the radar, much to my disappointment.
However, it seems to have finally gotten approval! So horay for that, and hopefully cosmology will finally be coming out of the dark, one way or the other.