What the astronomers are observing now is our galaxy having a speed of rotation in the outer arms which should be causing the stars in the outer arms to escape from the galaxy, but those stars are not doing so. The implicaton is that there is enough hidden mass in the galaxy to keep these stars in their galactic orbits.
Or that our understanding of how gravity works is incomplete.
I’m perfectly willing to consider the possibility of dark matter, and the even more weird dark energy.
But to assume as a fact the existence of something we cannot see in order to make what we can see fit the theories is not a very scientific approach.