Air carrier Captain reported a preflight DG documentation error which resulted in the aircraft departure with undocumented DG cargo on board.

Narrative

During preflight we received an ACARS message for dangerous goods loaded on the aircraft. When it was time for push back we had to request the final dangerous goods summary. At that time our push back crew said that their was no dangerous goods loaded. I told him that we have paperwork showing that DG was loaded and that we needed to get it sorted out. We then called operations and told them to call the load planner to verify whether we have DG on the aircraft or not. Again we were told that we did not have any DG on board. At this point I then requested the final DG paperwork showing the position (off) where it was originally supposedly loaded. I then got that paperwork and off we go. That should of been the end of it; but no. About an hour into the flight I get another ACARS message saying final dangerous goods summary loaded into position forward. So; now I have DG on my aircraft that I had no knowledge was there. I made a satellite call to Dispatch to try and find out again it there was indeed any DG loaded onto the aircraft or not. Dispatch called the load planner who told him that the DG was indeed loaded. My question is; how can we screw up something so major as not knowing what was loaded onto an airplane when it comes to DG.