If you have a .DAT data file of unknown origin, your best bet is to open it with a text editor before you try anything else. There may be clues to its program of origin in the text.

Index.dat file reader or viewer You can use freeware Index.dat Suite or Index.dat Scanner to view the index.dat file in Windows 7 or earlier. But you will not be able to use these freeware on ...

You can turn on the Show Hidden Files option by going to File Explorer > View > Hidden Items. You'll see that there is not only an NTUSER.DAT file, but also one or several more with a LOG extension.

File extensions usually tell your computer something about a particular file and how to open it. For instance, when the computer encounters a DOCX file, it will use Microsoft Word to open the file.