Did you ever wonder why you photograph black or white objects, and they come out all shades of gray or brown?  Here's a great explanation of why white and black colors are so difficult to photograph.

 

A camera meter tries to set an exposure setting based on an evenly balanced scene of lights and darks. If there is too much dark in a scene, it will overcompensate and the image will be too light; if there is too much white in a scene it will ovecompensate again and the image will appear too dark. The following image was shot on a half black and half white background using the same amount of light and auto exposure:

There is no such thing as your photos are too dark because you don´t have enough light. I could have tripled the amount of light on that last photo and the meter would still think "Whoa, way too light. I had better darken down a bit" and you still would get a too dark photo. I have taken photos in extremely dimly lit situations where I had to leave the shutter open for several minutes but still got a good photo.