It is in the translation exercise, but there's a problem with getting the machine to accept any spelling variation at all in the listen and type exercises, so unfortunately it's not accepted there. And since this is something that we can't fix for ourselves but must be done by Duo, nobody knows how long it will take.
The words de and dem are pronounced as though they were written dom: That's a somewhat informal spelling though, and in the 'listen and type' exercise, only one spelling can be accepted. So unfortunately you can spell it dom when translating, but not in the dictation exercises.
Silent letters are certainly found in a number of languages; however, what we see here seems a bit more unusual--maybe "ghost letters" (!). Is this addition of /m/ in the spoken form of "de" just particular to this one word, or does Swedish have a number of cases like this?