A file with the .dat file extension is a generic data file that stores specific information relating to the program that created the file. Some apps that use them are CCleaner, Porteus, and Minecraft.
DAT files are data files that contain information pertaining to the program that they're associated with. DAT files typically contain simple text, so they can usually be opened with a text editor like NotePad, TextEditor, or Notepad++, which is a free third party text editor.