Files with a DAT extension are data related.
They can contain anything from text files, graphics, and binary files in programs like a word processor, graphics programs you use, data media playback applications, or even accumulated data if you are browsing the web with Internet Explorer.
It depends on the program with which they were created and their content, the way they can be edited.
Some you can edit with a text editor such as notepad or notepad ++, others you can open with the application they were created (many times you don't know which one) and others that are binary or can be encrypted you can open them with a hex editor.
There are many free hex editors such as "HxD", "Radare", "Zynamics Hexer", etc.
I hope I've helped.