Theists are the one who gave the label to "atheists" and then they use that label as a scapegoat.
Non-believers are non-believers due to insufficient evidence of a god. They don't claim to know or prove that which can't be known.
Dark matter isn't something to "believe in". It's (if nothing else) a place holder. Galaxies require more mass than can be seen to stay together as they do. They need more mass to result in the gravity necessary to move as they do. Therefore there must be more mass somewhere. Dark matter is the placeholder for that. No faith required.
The point here is believe . We don't know a dam thing about dark matter . there have been experiments done and no thing was detected . There is a lot of hypothesis on hypothesis, and we are lead to believe there is dark matter and some explanation because it will satisfy the equation . the point we don't have instruments to detect it .. A believer in God have an inside feeling were science can not measure . How different is it for the sake of the argument .