A319 Captain reported returning to the gate to properly secure DG that the flight crew were not notified of being onboard. According to the reporter; Operations stated that the DG onboard did not need a notification.

Narrative

We pushed back late in ZZZ [Airport] due to late catering. As we were pushing; we received a flow time to ZZZ1 [Airport]. We still had not received our final weights so I thought it was OK that we had to wait for our flow time. We should've had plenty of time to get our final weights. The First Officer called Operations four times and was told that it should be any minute before we got our final weights. I sent a message to Dispatch while keeping Ground Control and the passengers informed. On the fifth call to Operations the First Officer was told that we needed to go back to the gate; because there were some hazmat that needed to be properly secured in the hold.I specifically remember seeing a no Dangerous Goods (DG)" message when we first initialized ACARS. We returned to the gate and shut down while the ground crew opened the cargo door and finished securing the hazmat. I called Dispatch to report the return to gate and find out what happened. I offered egress to the passengers and everyone wanted to stay on board. Someone in ZZZ Operations told us that this particular DG didn't need a notification. I still have no idea what it was. We were able to close the cargo door and push back within 15 minutes of returning to the gate. We got our final weights and departed for ZZZ1. We left ZZZ about an hour late.How could this possibly happen and why wasn't I notified that there were DG onboard?"