.dat files are just data storage files. dat = data. It can be near enough anything depending on what the maker of the program wishes todo.
You could try and search for legible strings inside the file, i would use UltraEdit. A very good all round program that supports hexediting.
Open it in that. Search for names you know exist in the game. Try replacing the name with a word of the same length. Put the modified file back over to the game folder and see if anythings changed. If you cant find any names or any farmiliar words. The file could be compressed or evern encrypted, or both. Dunno why itd be the second one though.
Id have to try it meself to know more about it.