Dryer duct: Conducive to mold or not?

So today I found a sizable amount of black moldish looking stuff in my dryer vent (the big tube hole in the wall) after the dryer machine was removed. The duct gets over half an hour of hot moist air blown into it every time the dryer dries, but I had figured that this would be a highly unsuitable place for mold since it is also still getting 10-30 minutes of dry air by the time the clothing inside the dryer are dry but the machine is going through its cooldown phase. Wonder how the mold stuff grows? Maybe the warm air condenses and becomes wet water in the duct?

Is it possible that moisture, etc could be making it’s way inside the vent from outside the house, since the dryer vents are routed to the outside? Maybe a type of flap or something isn’t closing properly?

I can’t see since the vents route somewhere else in the piping in the apartment that I know not of, but the mold in this instance is right behind the dryer- like literally mere inches or centimeters behind where the dryer was.

If the duct is blocked (lint build-up, or maybe, say, a birds nest) moisture can condense in the duct.

ETA - a good cleaning is always in order.

OP did not disclose his location.