If Everything is Spread Out, Where is the Central Mass that Everything Orbits Around?

There is no central object. In the solar system, there is - the Sun, but the planets don't just orbit the sun, they orbit "the sun and all of the other planets.'' But $99.99\%$ of the mass of the solar system is in the sun, so it looks like everything only orbits the sun. When things are about equal size, then both objects orbit each other, rather than one orbiting the other.