Black Powder Behind Center-Cap

My wife owns a 2000 Dodge Durango, it started making a popping sound from the front right wheel. In fact it looked like the center-cap was pulsating with each pop. Pulled the center-cap off and there is a ton of black powder behind it. What the heck am I looking at? Bad wheel bearing? Etc? Guess we need to get this fixed this week-end so that my wife and kids will have a vehicle for next week. Any help would be greatly appreciated.

Mike in Florida

After getting the caliper off, I can not for the life of me get the rotor/disk off. I have tried hitting it with everything you can think of to loosen it up with no luck. Am I missing something here, there are no lock washers on the studs. The rotor/disk was hanging up while it was turning, now it moves freely. I noticed there are teeth/gear on the back side of the rotor/disk. Help I'm lost, even with as many brakes jobs as I have done, I can't figure this one out.

Mike in Florida