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.

Although the data seemed to fit very well, I've never been terribly comfortable with the whole idea of dark energy.

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.