If the DAT file is a video, you can open it with its associated video app. However, if you aren't sure what app it's intended for, VLC Media Player should be able to open it as well.

Right-click the DAT file on your PC and click "Open." Click "Select a program from a list of installed programs" and click "OK". Click "Microsoft Excel" from the list of programs on your computer.

How to Open Multiple CSV Files in One Excel Spreadsheet. The CSV file extension appears on text documents that contain comma-separated values. Like spreadsheet data, each line in a CSV file ...