Any Unknown or Unlisted Aircraft Manufacturer
Air Traffic Control Developmental
Human-Machine Interface
Conflict NMAC
General None Reported / Taken
Aircraft
I was sitting on the D-side at 22/31/60 combined. R-side gave frequency change to Aircraft X. Aircraft X took frequency then quickly came back to say 'Center; we just almost had a mid air; did you see that guy?' To which the R-side responded; 'No; I see him now coming out behind you.'Aircraft Y was on a 1200 code; no data block information was shown; no traffic alert went off; and the target wasn't observed until after Aircraft X brought it to our attention. At the time of the event; the sectors were combined and fairly busy with lots of VFR traffic in the surrounding area.Split 22 out from 31/60 when there's moderate volume at 22.
ZDC Center Controller reported a NMAC with an aircraft they had just given a frequency change to and a VFR aircraft they did not see.