Actually, a: negative mass would have the opposite effect as dark matter - gravitationally repelling all nearby matter - so you'd need even more dark matter to

Actually, a: negative mass would have the opposite effect as dark matter - gravitationally repelling all nearby matter - so you'd need even more dark matter to make up for it; b: negative mass is actually attracted to positive mass[0], so quantaties of dark negative mass small enough to not have noticable gravitational effects would follow the same orbital trajectories as positive dark matter. 0: Eg, if you have two particles with equal and oppositely-signed mass in otherwise free space, they'll "fall" in the direction of the positive particle. The two-particle system has zero mass, zero kinetic energy and zero force driving its accelleration, which could be taken as a pretty good argument that negative-mass particles don't actually exist.