okay found an answer by Zmrzlina: "In constructions like "det regnar" (it's raining) and the like, where a subject is needed, Swedish always uses det. This works the same way with a demonstrative pronoun like det här. That's why it does not and does not have to agree in gender." https://www.duolingo.com/comment/6204238
Det här means "this" while detta can mean either "this" or "that". The specific way to say "that" is det där. Det här/där is more common spoken Swedish and I think detta is a bit more formal. Also, det här/där needs the noun to have the definite ending but detta does not.
T words: detta (hus) = det här/där (huset) N words: denna (man) = den här/där (mannen) Plural: dessa (kvinnor) = de här/där (kvinnorna)
But as far as I understand it (from the lesson and from the comments on the "det här är min fru" exercise), detta/denna is more formal when used in the form of "denna kvinna är min fru" (formal - this woman is my wife) vs "den här kvinnan är min fru" (informal - this woman is my wife), which is different from "det här är min fru" (This is my wife) which apparently didn't have a "detta" equivalent. What am I missing?