Dark Energy is the name given to the effect by which the Universe expands. In order to gain some appreciation of what it is that is causing this expansion (the Dark refers to the fact that we don’t know what it is, Energy refers to the fact that it provides the energy for expansion), surveys of standard candles are required. Standard candles are things like Type 1a supernovae whose absolute brightness is known to a good degree of accuracy, which can be seen from great distances and by whose apparent brightness (the dimming over a great distance of a bright light) distances can be measured.
Another way of measuring the distances in the universe is to measure the red shift – the Doppler shift of the light of objects in the far universe that are speeding away from us due to the expansion of the universe. In the case of constant expansion, the distances measured by red shift and those measured by supernovae should be the same. However, good, well measured supernovae aren’t necessarily available at all distances, especially not the red shift greater than 0.5 distances where non-linearities in the expansion are thought to be easiest to see.
A new dataset, called Union2, has been released to help. This includes a few new supernovae taken from Hubble Space Telescope data, some with VLT and Gemini data to boot, and a careful reanalysis of the Union dataset of supernovae. This provides new data in the 0.5-1 redshift region, which fits well with existing Dark Energy models. It isn’t extensive enough at the moment to make any ground breaking determinations between theories on what Dark Energy is and it doesn’t have much reach beyond a red shift of 1, but it is a step on the path of doing these things.