Dark scenes look terrible on my HDTV, whats the problem?

I own a samsung 46'' 720p lcd tv. Any time I watch tv, whether it is an HD channel or not, every time there is a dark scene, the picture quality looks spotty and unclear. I have the tv set to HDMI and purchased a HDMI cable. I cant really figure out whats going on, but when I watch a DVD or play video games I dont have have that same problem? Any ideas or suggestions?