If the DAT file you're trying to open is text, you can convert the file into text in the Microsoft Excel program by first saving it as a CSV file.

How to Automatically Convert an Excel File to Text Using Visual Basic. Microsoft's ubiquitous spreadsheet application, Excel, is a useful tool for small and large businesses alike.

A CSV file is a simple text file that you can open in a wide variety of programs, including any program that works with plain text like the Notepad app.