When it comes to prints being too dark, usually it is a result of an expectation of prints to appear exactly as they appear on a person’s monitor and not the printer itself. What this means is that the dark prints are really printing the way the image is but the users monitor is making a dark image look brighter. Being in the photo and fine art printing business, this is the one complaint that I hear the most.
This is most common for people using flat panel (LCD) monitors since they tend to emit brighter imagery than a more traditional and bulky computer (CRT) monitor. Laptops can be the worse culprit in all of this. MAC users may also experience this issue due to differences in the Gamma curve.
The easiest test is to print an image on your desktop printer at home on a decent photo quality paper.* If your print comes out looking dark then most likely your monitor is simply making your image too bright. The solution is usually a simple one if you are able to use a simple image editing program. Most let you adjust your pictures contrast and brightness levels. Run another test print and see if that does the trick. More more on how to test your images before taking them to print can be found if you click here.
You may also want to consult your monitor or video card settings to see if you can adjust the brightness and contrast of your monitor to match closer to how your printer outputs pictures.
*Note that various factors such as printer, media brightness, texture and size can affect actual color tones and perceived color as a result of color density.