To promote my own tool, https://github.com/bradfitz/shotizam lets you drill down into why Go binaries are large without having to make up terms like "dark bytes".
Because it's genuinely unknown what those are, figuring out what these bytes are is just a deep dive into the opensource go compiler at worst or just asking around on some go channels for much less effort.
This code, if I'm reading it right, uses the symbol table and ELF section headers.
As explained in OP, the sum of sizes advertised in the symtable and ELF section headers does not add up to the final binary size. The shotizam tool is thus blind to that difference.