CRJ-900 First Officer reported smoke and odor in the cockpit during departure taxi. Flight executed a ground gate return and taxied into gate without marshals who were non-responsive.

Narrative

It was my second trip of OE as FO (First Officer). We were doing a leg ZZZ to ZZZ1 and it was first flight of the day. After starting right engine we decided to shut off the APU as there was some planes sequenced ahead of us and he could also demonstrate a cross bleed start. Right as we were departing the ramp area the Captain made a comment about smelling an odor/smoke; but there was nothing visual. At that point I didn't smell anything and we continued taxiing. We were cleared to cross a runway and when power was added to break away; the cockpit filled with smoke and the odor to me was apparent. We weren't sure where the smoke came from; it seemed as if it came from everywhere around us. The smell of the smoke I couldn't determine what it had come from (electrical; oil; plastic; etc...). The smoke dissipated but we were unsure on if the cabin experienced the same situation. After crossing the runway we had switched to Tower frequency and asked to have some time to run checklists. As we were blocking the routing to the runway they had us turn off the taxiway we were on. After coming to a stop we ran though the smoke removal emergency checklist and proceeded to the QRH. There were no EICAS messages to go off of for any other checklists to run. The Captain talked with Dispatch and the Flight Attendants afterwards as I tried to get us a gate to return to. After we had things squared away we advised the Tower that we were ready to return to X Gate. As we taxied back the smell lingered and a very faint smoke remained. As we were approaching the gate we were delayed from a departing aircraft from the ramp area. Once clear; we were lined up for the gate; but had no marshalers. The Captain had talked with Operations a couple times at this point about needing marshalers. We weren't given an ETA on how long it would be for marshalers to arrive. At that point the Captain stated that if we don't have marshalers in a minute he was going to taxi it in without any due to the smoke still in the cabin. That minute passed and no marshalers appeared. At that point he stressed keeping my head up and looking out for anything on my side as we taxied in without marshalers. Once we were at the gate we shut down and had the FA (Flight Attendant) open the door to air out the cabin. The Captain pulled in a ft. or two too far for the gate and had to get a tug to push us back for the jet way to be connected. After which; the aircraft was deplaned and we were assigned a new plane.Cause of the scenario was smoke in the cabin; leading to a gate return. Taxi without marshalers was due to slow/lack of response to the situation that was explained by the Captain. Cause of the smoke unknown.