CLOSE ENCOUNTER BTWN ACR MLG X AND UNKNOWN VFR TFC Y ON NIGHT FLT.

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THE FO OBSERVED A LIGHT OUT TO THE R. I MADE EYE CONTACT WITH IT AND OBSERVED IT TO BE APPROX 2 O'CLOCK AND HORIZ TO US. IT APPEARED TO BE MOVING FORWARD AND CLOSER TO US. THE PF ABRUPTLY PULLED UP (DISCONNECTING THE AUTOPLT) AND CLBED APPROX 2000 FT. DURING THE CLB I WATCHED ACFT Y PASS DIRECTLY UNDER US. THE CTLR THEN CLRED US TO DSND TO 10000 FT. WE STARTED DSCNT AND ASKED ATC IF THEY HAD SHOWED A CONFLICT. ATC STATED THEY DID AND THAT IT WAS A VFR ACFT Y AND THAT THE CTLR WAS UNABLE TO TELL US. WE DID NOT HAVE TCASII INSTALLED. IT WAS AT NIGHT SO WE WERE ABLE TO SEE THE ACFT'S LIGHT. THE VFR TFC APPEARED TO BE FLYING JUST ABOVE THE CLOUD LAYER. SUPPLEMENTAL INFO FROM ACN 204690: IN THE NIGHT CONDITIONS THE SMALLER ACFT Y LOOKED TO BE MUCH FARTHER AWAY THAN IT ACTUALLY WAS. SUPPLEMENTAL INFO FROM ACN 203147: NEAR MISS WITH UNKNOWN VFR ACFT Y VICINITY IGN VOR; APPROX 13800 FT ALT. WE CLBED RAPIDLY TO AVOID CONTACT. ATC SAID EQUIP DISPLAYED CONFLICT BUT WAS NOT RELAYED TO OUR ACFT IN TIMELY MANNER. HENCE; VIGOROUS LAST MIN CORRECTION INITIATED.