As DAT is a raw file format that is not associated with any application, so there is no dedicated DAT file viewer application to download. A DAT file can contain practically anything, including documents, video, pictures and music.
Most file formats are associated with the application that creates or reads them. For instance, the DOCX format indicates that the file is a Microsoft Word document, or an the AVI format indicates that the file contains a video that users can open in a media player application. However, DAT files may contain anything and thus be compatible with any application.
Windows hides the DAT extension by default. To see it in the file name, click the Start button, open Control Panel, and then click Appearance and Personalization. Click Folder Options, navigate to the View tab, and turn off the Hide Extensions For Known File Types setting. You can now see ".DAT" next to the name field of a DAT file.
Without a dedicated application, you must guess the contents of a DAT file to open it. One way to do so is to check its size. Right-click the file, and click Properties. If the file takes up hundreds of megabytes of storage space, it is likely to be a video file. If the file size is around 10 megabytes, it may be a song. If it's any smaller, the file may be a short document.
Alternatively, consider where you got the file. If it was in a VCD, the DAT file is very likely to be a movie. You can also open the file in Notepad, press F while holding CTRL, and search for application names. For instance, if you see "Photoshop" written in the file, it's very likely that the previous user created the file in Adobe Photoshop.