B737 First Officer reported the aircraft had to return to the gate due to DG on board. The flight crew never received DG paperwork but was later informed that a low amount of dry ice did not require paperwork.

Narrative

Normal preflight; pushback; and engine start. While waiting on final weights we contacted Ramp to give them an update; to which they replied that Operations was trying to get a hold of us for a gate return. We contacted Operations to get our gate assignment and were told we had to return due to a 'loading issue with Dangerous Goods (DG).' We had not received any DG paperwork during preflight. We pulled back into the gate and were told that the ramp personnel were going to get into the cargo hold and 'move some items around.' We voiced our concerns about the lack of DG paperwork. After an hour and change of delays; including pulling a jetbridge up to allow for passenger egress; while talking with Ramp; Operations; Dispatch and Load Planning we finally were told that the lack of DG paperwork was due to the low amounts of dry ice. While this is in the FOM; no one could confirm the amount of ice in each package. We were not comfortable continuing with the flight until someone could give us a solid answer about the nature of the DG onboard.