You're not the first, second, or ten thousandth person to have this thought. But dark matter fits the phenomena better than any proposed modification of gravity. See the https://en.wikipedia.org/wiki/Bullet_Cluster for a textbook example.
Also, look at the Bolshoi Cosmological Simulation (https://en.wikipedia.org/wiki/Bolshoi_Cosmological_Simulatio...). Simulating the universe with the currently accepted model of dark matter successfully predicted the large scale structure of the universe. As a model for explaining observations, dark matter does hold some merit.