Dark matter is an assumption to explain galaxy shape using gravity only.
What about electromagnetically charged plasma? That force would produce observed shape and does not require imagined matter.
An electrically charged plasma would interact in known way with light coming from stars in the Milky Way. Those interactions are not observed, so no plasma.
What IS observed is that galaxies don’t rotate the way that should based on Newtonian gravity or General Relativity. Either our gravitational theories are wrong or there is some unaccounted for mass screwing up the calculations. Both of these possibilities have been investigated (Google MOND - modified Newtonian dynamics of you want to find out more), and reality matches better with the unaccounted for mass, I.e. dark matter.
I’m not really sure why massive particles that only interact gravitationally is such a problematic idea anyway. There are only four possible ways particles can interact—electromagnetic, strong, weak, and gravity. We already have detected particles that don’t interact via the strong and electromagnetic forces, namely neutrinos, which interact only via the weak force and gravity. Is it really such a stretch to think there might be neutrino-like particles that don’t interact via the weak force?