A file with DAT file extension is a Data file.
The simple way of open this type file is double click on it and let your system decided it which is the default program for it. If the window does not open this file then you have no install any application to view DAT file.
Tip:- In this situation, firstly try to open it with the help of word pad and any text editor, Depending on where the DAT file came from, it could be a generic text-based file so this trick is worth a try. If it does not work, then you have to convert DAT file in another file extension like PDF.
For more info about conversion process read this http://www.ehow.com/how_7857641_convert-dat-files-pdf.html