You make some observations that existing theory can't explain
So you invent a new concept to explain these observations
This is a hypothesis
Now you have to go do actual science and test your hypothesis
You have to understand this thing you've postulated and explain it

"Dark matter" as a stand-in for unobserved matter that is hypothesized to affect gravity is totally fine and scientific
But it's not a theory or law of science or any such thing
And treating it as a real, known entity on par with regular matter is in fact deeply unscientific