Centrum does not have a centrumen/centrumet version?
I would use "centeret" instead of using "centrummet". It sounds clumsy, though Wikipedia says you can: https://da.m.wiktionary.org/wiki/centrum
I'm confused. uksually the is 'en' or 'et' on the end of the word. Now it's um as the accusative in Latin. It sounds cool but how did that happen?
The same way as other Germanic languages, just take a look at German and Dutch. :-)