The Difference Between Extra Gravity and Dark Matter

If it's a tweak to the equation, then it has to be everywhere, right? Universal. But if it's stuff, if it's dark matter, it could be absent in certain locations. It could be someplace, not another place. So one way is to see if there are any observations of things happening out there in the universe that can be explained if this extra gravity is being produced by stuff rather than just an equation.