> Galaxies behave like there's a halo of invisible mass surrounding them,
No they don't. Galaxies exhibit anomalous velocity curves according to existing gravitational theory as applied in a specific cosmological model. That's the observation. This could be explained by a halo of indivisible stuff or it could be explained by a misunderstanding of gravity or a different cosmological model.
I'm general, we should not explain observations in terms of speculative theories, because this carries a presumptive bias.
> The halos even perturb as expected in galaxy collisions.
They actually don't. Dark matter would explain part of the bullet cluster (lensing), but it can't explain the high collision velocities observed.
No they don't. Galaxies exhibit anomalous velocity curves according to existing gravitational theory as applied in a specific cosmological model. That's the observation. This could be explained by a halo of indivisible stuff or it could be explained by a misunderstanding of gravity or a different cosmological model.
I'm general, we should not explain observations in terms of speculative theories, because this carries a presumptive bias.
> The halos even perturb as expected in galaxy collisions.
They actually don't. Dark matter would explain part of the bullet cluster (lensing), but it can't explain the high collision velocities observed.