What were you expecting to see in a text editor when opening this file? .dat files are generally data, specific to whatever application generated them. Often, they are binary – and it obviously looks binary based on your paste.
If you want to be able to edit some underlying textual data, you will have to determine what application created the .dat file, and find out whether there is a converter from that format to plaintext (it might be a converter, or an unzip-like utility, or a decompiler, or…). Once you have it in a text format, it will be easy to edit it with Notepad++. Once edited, you can use the same (or complementary) converter to get it back to the original format.
Notepad++ isn’t some magic program that can “read any file format, convert it to text for editing, and convert it back to the original encoded format when saving”. It’s an excellent text editor, and with the associated scripting and assuming an appropriate converter, it would be possible to automate the process of convert-on-load, convert-on-save, but it’s not going to be able to take an arbitrary binary format and magically know how to decode/decompile it.