Air carrier Captain reported a communication breakdown between the flight crew and ground personnel regarding missing DG documents during preflight. After some research it was determined that the onboard DG was low risk and exempt from the required DG documentation.

Narrative

During final preparations for push back ramp crew advised the crew that due to a lack of checked bags they had to load 450 pounds of ballast to secure a package with Dangerous Good (DG) in the cargo bin. This concerned us because we did not receive any notification via ACARS. Both ramp and ZZZ ops confirmed we did have DG on board and when we contacted Dispatch and Load Planning they were not showing DG. At this point we paused things to ensure things were figured out. Load planning was showing the requirement for the Ballast to secure the DG. When we did initially contact load planning both directly and via ZZZ ops we were told that DG info would come with final weights; reinforcing the need for DG paperwork. When they did send final weight it was not mentioned and we got 3 no dangerous goods planned" messages. This I can only assume was load planning attempting to send us DG paperwork. At that point ops couldn't figure out what was wrong with the DG process and was going to pull the DG. After some digging it was revealed by load planning that the DG was Bio Exempt and DG / Hazmat paperwork was not required (supported by Maintenance Facility X.XX.X). Once that was revealed we did depart once confirming everything was proper."