The .DAT file format is a simple data file that could have been created by just about any software available. Although any text editor will probably be able to open a .DAT file, the precise format of the file will depend on the program that created it. The only way to view or access the information correctly will be to establish which software generated the file and then to use this to access it.
If it isnt immediately obvious which program generated the file there are a few tricks you can use to identify the software. Firstly you can search through the file using a text editor looking for any hints about the original generator. Alternatively the folder in which the file is sits might also give an indication, as well as the time and date the file was created. Since the text editor will only show you the contents of the file, but will not necessarily restore the formatting correctly, be careful not to save the opened .DAT file since this might render the file unreadable to the program that created it. Specific software that can open .DAT files include the program that created the file (if known), and any text editor.