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the 3 black wires twisted together, with the white wire, are the constant feed. the white wire, along with a black wire goes to a switch. Im guessing you had the white conductor from the 2 wire going to the switch connected to the fan, additionally you would have your neutral wires spliced together with a whip to the fan. Usually when a switch is installed the power comes down on the white wire, and returns to the light box on the black wire. This is one of the only times you can use a white conductor as a hot wire. you are suposed to re identify it as such though, with either tape or a permanant marker.
Without a voltage tester, it's tough. Here's what I'd do. Hook you light to the white bundle and one of the black bundles. If it comes on, see if the switch turns it off. If no, try the black/white bundle. One of the two should be the switched source. If you do it hot, be careful and let go before the shock gets to your elbow.:D
dont you just love electrical makeup in cieling boxes? dont you just love white (neutral) wires used as switch legs? back fed swithces? Take a tester (not your forefinger and thumb) with the switch off one set of your wires should be dead. turn the switch on, one set should light up. (tester,not you) Attach that to the black wire of your fixture. turn the switch off one set of your wires should be hot,cap these and leave them alone. that leaves one set of wires that should be your neutral,connect them to your white fixture wire. hope this isnt too convoluted.