DAT files are data files. You can open .DAT files that contain text with text editors, like Notepad, Notepad++, VS Code, etc.

Right-click on the .DAT data file in question and move your mouse cursor over the "Open with" list item in the context menu. A submenu will now be displayed.

If you're having trouble opening a DAT file, I'll show you a simple and free way to do it. This method works on both Mac and PC. Just open your browser and go to winmaildat.com. It's a ...

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.