It is not. Dark matter is not some smooth distribution that could be explained by adjusting the formula for gravity. There are places where it is, and places where it isn't. Not only that, but it has its own mass and inertia.

Not only that but dark matter is missing in a host of other ways that can't be explained by the laws of gravity being more complex. Microwave background radiation is 30x more powerful than can be explained by visible matter, but matches exactly if you add in dark matter and dark energy. The ratio of hydrogen to deuterium is inconsistent with visible matter, but matches perfectly if you add dark matter.

Galaxy clusters collide and we can literally see dark matter pass through the collision since it doesn't interact.

Dark matter is as well proven as the atom was when we made the bombs. We just don't know what it's made of yet. You simply don't know enough about it.