I am rephrasing an answer to another PSE question as an new question here.
Dark matter is invoked to explain the 'observed space-time geometry that does not match with predictions'.
According to the Einstein-Cartan version of gravity, there are two sources which "distort" local space-time:
- Mass (or more generally energy-momentum tensor), which induces space-time curvature.
- Spin (spin current from Dirac fermions), which induces space-time torsion.
As an alternative to missing/dark mass, can we attribute the "observed space-time geometry that does not match with predictions" to dark spin current?
As per Poplawski, the non-zero spin/torsion approach has another nice property that "averts the unphysical big-bang singularity, replacing it with a cusp-like bounce at a finite minimum scale factor, before which the Universe was contracting".
As an alternative to dark matter, the MOND (modified Newtonian dynamics) hypothesis explains nicely the flat rotation curve of a spiral galaxy (Tully-Fisher law). But MOND failed miserably at inter-galactic scale and has nothing to say about cosmological scale dark matter, though there are some stilted versions of relativistic MOND extension. So "dark spin current" may come to the rescue.