No 'thinking' about it, that is exactly WHY they came up with "dark matter".
The 'estimated' MASS of a Galaxy was not enough to exhibit the gravitational force that we perceive based on the movement of objects at the outer fringes of a galaxy. Therefore there had to be more 'mass' that we simply could not see/detect.
“The ‘estimated’ MASS of a Galaxy was not enough to exhibit the gravitational force that we perceive based on the movement of objects at the outer fringes of a galaxy.”
It’s really even worse than that, because if you just increase the mass of the galaxy but keep the same mass distribution, then the motion would still not fit the equations. In order for “dark matter” to have any effect at all, it must be distributed in just the right spots so that it could alter the motion from what the equations predict to what we see.
So the “dark matter” must know what equations we will be using to try and predict the motion of galaxies, and then decide to distribute itself in such a way as to foil us. Really, I don’t think scientists realize it, but they are basically proposing that “dark matter” is intelligent and omniscient!