The "darc" file format was found in the early version of Nintendo Ware for CTR (nw4c).
This tool can help you extract existing darc files and create new darc files.
Extract "input.arc" to "output" folder:
python darc.py -xf input.arc -d output
Change directory to "input/" and add entries recursively to "output.arc":
python darc.py -cf output.arc -d input dirs files ...
Create darc with all file data align to 32:
python darc.py -c -a 0x20 -f output.arc -d input dirs files ...
Create darc with *.bcfnt files data align to 128, other files align to 32:
python darc.py -c -a 0x20 -t *.bcfnt:0x80 -f output.arc -d input dirs files ...
By default, this tool creates archives without the "." entry, which is commonly found in many games. If you need that entry, you just need to add a "." as the input entry. For example:
python darc.py -c -f output.arc -d input .
Note that there's a dot at the end of the command