Originally Posted by Ken G

But I think what you are really asking is, is it possible that dark matter, whatever its explanation, might only interact with the known universe gravitationally, and in no other way? To that I would say, anything's possible. But note that if this is the case, it's a pretty major crisis for astronomy-- there's only so much you can do by mapping its gravitational influence, and after that it would never get past hypothetical possibilities. We'd like an independent way to explore the properties of that stuff, such as a laboratory detection, or some type of astrophysical observation that would be directly attributed to its decay into things we can detect.