You're are implicitly assuming something that cosmologists do not assume: that DM even exists. It is not assumed to exist. It is proposed to exist as an explana

You're are implicitly assuming something that cosmologists do not assume: that DM even exists. It is not assumed to exist. It is proposed to exist as an explanation for observations we have made. As such, the proposing scientists are free to set the terms of their own proposal. The whole reason DM was proposed is because, if true, it explains gravitational phenomena of galaxies that we can observe but can't otherwise explain under the current best theory of gravity. (Some alternative proposals include very different theories of gravity, rather than DM. [0]) The weakly-interacting property of the proposed DM (again, this property is not an assumption; it's part of the proposal) is what leads to the spherical distribution. It is a (very well explained) mathematical consequence of the lack of interaction that the DM retains a spherical distribution while the ordinary matter collapses to a plane. [0] https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics