Circular velocity, vcirc, as a function of radius, r, as extracted from the N-body simulations. The rotation curves shown are: (i) for a positive mass galaxy consisting of 5000 particles (in black), and (ii) for an identical positive mass galaxy with identical initial conditions, albeit now surrounded by an initially uniform distribution of 45 000 negative mass particles (in red). The rotation curves are plotted using a moving average for each set of 50 particles – the means and standard errors in υcirc for these particles are indicated by the solid lines and the shaded areas respectively. The rotation of the galaxy that consists of only positive mass clearly drops off following a Keplerian curve. Nevertheless, the galaxy with a negative mass halo has an essentially flat rotation curve that is slowly rising out to large galactic radii.