Assumption, guess, educated guess, prediction, theoretical condition... you could call it many different things.
Possible, perhaps. But we're talking about a LOT of missing mass since something like 85% of the mass of the universe is dark matter. It can't all be cold hydrogen-helium gas otherwise we would see it in the rates of stellar formation, absorption rates of various EM frequencies, and other things. It can't all be dust or we would see it obscuring background targets, measure different stellar metallicity, and in other ways. Having it made up of sub-stellar matter (planets, asteroids, comets, etc) doesn't fit with either observations or theoretical models of how star systems form, where the vast majority of the mass is in the stars and only a small fraction is contained in the sub-stellar objects surrounding the stars. We've catalogued a lot of star systems and none of them have hugely abnormal amounts sub-stellar matter.
These problems continue when you consider all other known objects or types of matter. There just isn't a way that we've found to reconcile the behaviors of galaxies and galaxy clusters with missing 'normal' matter. The observations just don't support it.
That's not to say that dark matter is a perfect candidate. It's not. It's just the best model we have right now.
Also, remember that we already know of matter that interacts via four, three, and two of the fundamental forces (quarks, electrons, and neutrinos respectively, as an example of each). Is it THAT much of a stretch to imagine a type of matter that only interacts via gravity?