This has been asked 3 times in as many days (not necesarily a bad thing). Here is an excellent answer by u/alricofgar. Just some more info, many historians are moving away from the term "dark ages" due to a lot of common misconceptions people have about the term. For instance, many people think it refers to the entire middle ages and all of Europe, when it only really refers to about 400-900 and mostly in the British Isles.

An great book to learn more about the period is Britain After Rome: The Fall and Rise 400 to 1070 by Robin Fleming. Fleming talks about the state of Britain after Rome withdrew and how that affected the culture, economy, religion and also goes into depth about where we get this information.