I'm wondering what is the dark area on my dental x-ray?

Empty space: Wherever you see total dark area on xrays, there is nothing there. That is wt decay shows up as a darker spot, since decay is thinner than the surrounding tooth strcture and xrays pass through that area more resulting in a darker area, versus if you have a silve filling or a metal crown in your mouth, that shows up as white, since xrays do not pass through them.