There are times when you might receive files with unknown extensions, one of them maybe a file with a .DAT extension. DAT files are basically data files which can be opened with different software, however, it is very tricky to decide what to use.
One of the most common DAT files is Index.dat, which is a Internet Explorer history file which stores your browsing history.
Another common type of file a .DAT file could be CSV or Comma Separated Values. This can be opened with either notepad or any other text editor along with Microsoft excel. Video files found on CDs and DVDs also use the DAT format very often.
You can also try opening DAT files on Windows Media Player or any other video player you use. Other than that you can try to open DAT files using notepad or any other text editor.
Again DAT files contain arbitrary data so it really hard to tell what data is contained within it. As always try using notepad or another text editor to open the file or use Windows Media Player.