A "--" in the district column means, that the town is a district-free town, i.e. it is by itself a district.
... size is 2.38 and the average family size is 3.02. In the town the population is spread out with 22.3% under the age of 18, 8.0% from 18 to 24, 31.0% from 25 to 44, 21.9% ...