You need a program that is compatible with opening dat.files. It depends what the file is as well.
Quote by Twist of fate
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Dat's da first thing I thought of when I saw this thread
open it with notepad.
Quote by PluckU
OH **** YOU says the computer, YOU MAY ONLY HAVE 1, READ IT, ONE AUDIO TRACK >:O I then i got pissed off and through my tuner against a wall
what are you trying to open? Is it something you downloaded using torrents or something?
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.dat is such a generic filetype, and can store any sort of data. Where did you download it from? What for? Why are you trying to open it? etc. There's no one programme.

More often than not .dats stores plaintext data that for one reason or another programmers don't want users to edit manually for fear that they'll corrupt whatever formatting it has.

The last time I saw a .dat file was 5 or 6 years ago when using a bastardised version of Cutenews.

So yeah. Context is everything.