Dark backgrounds are generally considered to be less readable than light backgrounds. A sufficient level of contrast is also very important to readability.
In general, I would recommend dark backgrounds for designs that have a large amount of media content, but very little text. Darker backgrounds can really make photographs stand out and you’ll find many photography sites will have a black background.
If you have a lot of text, however, you should use a light or white background. It’s easier on the eyes and it mimics what we’re used to when we’re reading newspapers or books. You can also use light or white for media content, but it has a less artsy feel.
For further reading: Light text on dark background vs.