To open a DAT file on any device, first identify its content type—DAT files are generic and can contain video, text, or data. On Windows or Mac, try opening it with a text editor like Notepad or TextEdit if it's text-based. For video DAT files (e.g., from VCDs), use VLC Media Player. On mobile devices, use a file manager app with built-in viewers or VLC for media. If the file doesn't open properly, check its origin or file association to choose the right app.