Black Mass - Hated It

I am not a fan of movies like this. Mafia-oriented, dark, slow moving, all of these describe this movie. It could not keep my attention in the least. Honestly, I'm not 100% sure what this movie is about. I'm assuming the chase of a fugitive who was the leader of the mafia, because that's what it looked like, there were just too many characters to follow. Who were the bad people (other than the main character), who were the police, who were the undercover cops? I just don't know. The ending made me think that this movie was based on true events, but couldn't tell you if that is true. 

Let's talk about the casting... I had to look up who played the characters, and OMG I had no clue that was Johnny Depp as the leading role. My mind was seriously blown, the entire movie I was wondering who that guy was and how old he was because he just looked... awful. Dakota Johnson as his wife was off to me, though in what little scenes she was in I thought she did a pretty good job. Like I mentioned before, it didn't keep my attention at all, and was confused when all of a sudden Kevin Bacon and Benedict Cumberbatch are in the story. I think as FBI people. 

Sorry if my review isn't helpful if you are trying to make a decision to watch it or not. I tried to watch it, I really did. Overall I hated this movie and had no urge to continue watching it. I don't even understand the title, can someone help me out by explaining this? Don't watch this unless you're into this sort of movie.