Bigfile.000 is an archive format (often using the .tiger extension in newer titles) used to bundle thousands of smaller game files into a single manageable container.
If you are certain the file is a single archive or disk image incorrectly labeled .000 : Unable To Open Bigfile Bigfile.000
If any number is missing, the image is incomplete. In some tools, even a missing final segment (which might be smaller) will cause the first segment to fail to open because the tool pre-scans the chain. Bigfile