Atheists;Is the "dark matter" theory based on anymore than an umption?

I'm not atheist, but I can tell you that the theory of dark matter is based on the observation that there is not enough detectable matter in galaxies to account for their motion in relation to their gravitational fields. Thus, they postulate undetectable (or nearly so) matter. It is not unreasonable, but is also not based on anything more than a hypothesis at the moment.