NMAC BTWN A CARGO DC10-30 CLBING ON DEP AT 11500 FT AND A CESSNA SEL CONVERGING AT THE SAME ALT. NO EVASIVE ACTION TAKEN DUE TO INSUFFICIENT TIME. ATC ACKNOWLEDGED TFC AT THAT ALT AFTER BEING ADVISED OF NMAC BY RPTR'S CREW.

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ON A RADAR VECTOR DEP WITH AN UNUSUAL NUMBER OF INTERMEDIATE LEVELOFFS; WE WERE ON AN ASSIGNED HDG OF 070 DEGS AND CLRED TO CLB OUT OF OUR 10000 FT ASSIGNMENT; OUR NEW ASSIGNMENT WAS 13000 FT OR 15000 FT. OUR CLB AIRSPD WAS 320 KIAS. APCHING 11500 FT IN THE CLB; MY PERIPHERAL VISION CAUGHT MOVEMENT OUTSIDE MY R WINDSCREEN. TOO LATE TO MANEUVER; I WATCHED AS A CESSNA-TYPE ACFT; WHITE WITH RED STRIPES; PASS AN ESTIMATED 200-300 FT FROM MY COCKPIT WINDOW ON A SLIGHTLY CONVERGING HDG. WHEN WE RPTED A NEAR MISS WITH ATC; THE CTLR RESPONSE WAS '...YOU MEAN THAT GUY AT 11500 FT?' THIS CONFUSED US EVEN FURTHER. THE CTLR COMMUNICATED TO US THAT HE KNEW THE ACFT WAS THERE BUT DID NOT SOUND AS THOUGH HE FELT ANY CONFLICT HAD EXISTED. THE CTLR NEVER ADMITTED ANY ERROR ON HIS OR HIS EQUIP'S PART NOR XMITTED ANY CONCERN FOR THE GRAVITY OF THE SIT. I WISH WE COULD SHOW THIS GUY HOW NEAR DEATH WE CAME THAT DAY. THE HIGH NUMBER OF INTERMEDIATE LEVELOFFS COULD INDICATE A COMPLICATED ARR/DEP SIT WITH MULTIPLE ACFT CAUSING HIGH TASK SATURATION. THIS INCIDENT REINFORCES THE NEED FOR AT LEAST 1 PLT TO VIGILANTLY WATCH OUTSIDE THE COCKPIT FOR TFC; ESPECIALLY DURING DEPS AND ARRS.