MX (.mx) is Wolfram Language serialized package format developed by Wolfram Research. It stores arbitrary Wolfram Language expressions in a serialized format optimized for fast loading and preserves anything including the options, etc. For example

res = RandomGraph[{15, 35}, VertexSize -> "BetweennessCentrality"]

Save the fale with DumpSave

DumpSave["res.mx", res];

Now relaunch the kernel:

Exit

And reload the file:

<< res.mx

You will not see result but you have the definition of your variable:

res

Keep in mind that MX files cannot be exchanged between different operating systems or versions of the Wolfram System. In this case you can Export compressed string of any expression:

Compress[res]
Export["res.txt", %]

and Import it back intact:

Uncompress[Import["res.txt"]]

I'd recommend careful reading through: How do I save a variable or function definition to a file?