The paper uses the version where both gravitational and inertial mass is negative. This means that everything wants to accelerate towards positive mass and away

The paper uses the version where both gravitational and inertial mass is negative. This means that everything wants to accelerate towards positive mass and away from negative mass. So positive mass naturally forms clusters (galaxies) and negative mass naturally spreads out evenly but concentrates around the galaxies (forming the dark-matter halos).