You asked for good references. Here are three. I don’t know your level of sophistication, but these are written for a funder’s point of view and are hence easier to understand. But the many references contained therein will give you a more detailed science approach if that is what you are interested in.
The first reference, from around 2005, is still regarded as the definitive statement of the problem from a US point of view. The second, from 2012, is a very good update from one specific project perspective (LSST). The third, a website, gives you the current take from the leading European related space project (Euclid).
You also asked why this is important. Everyone seems to agree that the dark energy is the most massive unsolved problem in cosmology. Let me quote from the top of the third reference:
“Theme: How did the Universe originate and what is it made of?”
Is that important enough for you? (Grin)