Many applications create files with extension ".dat" (and others, possibly including your ".lst") in which they store private data. Sometimes this is in text format so that you can read it; sometimes it is in a private binary format.
To understand a binary format ".dat" file you will need
(1) a binary file editor, and:
(2) either (a) a specification of the file format or (b) some experience at reverse engineering binary file formats (plus some amount of luck).
If you know what application created the file you can usually find that someone has already done the reverse engineering for you and posted a specification of the file format somewhere on the web.