Dark matter is posited because of the difference in what was expected and what was actually captured by the scientific equipment.. Spirits on the other hand, are usually just anecdotal things with no evidence other than what people say they've seen, like the Loch Ness monster and Yetis. Until there is some actual evidence it can be lump[ed along with all the other myths.
You can fix ignorance, you can't fix stupid.
Tinkety Tonk and down with the Nazis.
