It's not missing though, is it? It has the characteristic of having effects of mass but not being otherwise observable.

Other comments have suggested better names, based on the fact that we don't even know for sure that it's matter, just that gravity at large scale doesn't behave like it should.

If you say "missing matter", the thing most physicists think of first is probably missing anti-matter, and then missing baryons. Not dark matter.

It's a bit late to suggest new nomenclature for something that's been around for many decades.