You can open .DAT files that contain text, with text editors, like Notepad, Notepad++, VS Code, etc. But it is not possible to know what contains in a DAT file.

Just open your browser and go to winmaildat.com. It's a straightforward website that lets you upload and open DAT files without needing to sign up or provide an email.

If you have a .DAT data file of unknown origin, your best bet is to open it with a text editor before you try anything else. There may be clues to its program of origin in the text.

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.