John, welcome on macosx.com
Well, .dat extension is a generic name...
If it is a ASCII text file, BBedit Lite
http://www.macupdate.com/info.php/id/3693
can view it
If it is an image, GraphicConverter can do it
CanOpener
http://www.abbottsystems.com/
is dedicated to open all kind of file.
So, now, what do you want to do with this file ? Is it a virus definition database ?
Does it come from Windows world ?
Regards
Philippe