...depends on what you are shooting and how you are metering. For low key images that have lots of dark areas, a left-weighted histogram is normal. For high-key images, the histogram will lean the other way. For midtoned images, the histogram will max in the middle somewhere. And for contrasty images, you might find the middle of the histogram empty and the left and right sides full. Etc. etc.
So, if your images are indeed not low-key but simply appear to be too dark, you need to add some exposure compensation to push the histogram to the right, if that is the effect you want to achieve. Many D100 posters on this site and others are reporting routinely dialing in +0.7 EV of compensation, but I find this will blow out highlights in the typical shots I make. (I think Nikon may have made the D100 err on the low side of exposure for exactly this reason.) Quite frankly, I have been dialing in -0.3 EV to hold back highlight burnout for much of my shooting, but for some mid-toned shots I can see the need to dial in some + exposure compensation, especially when using matrix metering. This medium is different than film, and I am busy un-learning many ingrained film instincts with the D100.