Or maybe the effect comes from the flying spaghetti monster? No, the point is that scientific ideas (dark matter, gravitons, cosmic strings) come from theories which have other scientific justification -- if various other, unrelated, ideas from particle physics such as supersymmetry are true, then all of these things are predicted to exist. Moreover, just because these entities can't be observed today, that doesn't mean that they are in principle unobservable. This is very different from the god of (say) Christianity whose existence seems to require my death to check, an experiment that I, for one, am unwilling to perform.