If I have run this idea down in the past, well, forgive me. Hey, I'm old.
I have a rather naive idea about dark matter. I don't really know the physics. I was a physics major in college, but that was 50 years ago, and it has...changed over the years. I wonder what anyone else thinks of it, and more to the point, is there something about current physics that makes it moot.
We all know about flat land - the two dimensional land that could be my table top. people there have no awareness of "above". as seen from up here, people look like little circles. Now imagine a big bowling ball of lead suspended a millimeter above the center of the table - of flatland. It will have gravity. All around the table, the folks will be gravitationally attracted to the center of the table. Oh it will actually be a vector angling slightly above, but they are not aware of that they only feel the component in their plane. The ball is dimensionally removed from their experience. One special aspect. As they get close to the center, that vector angle gets larger and larger. SO the component in their plane gets smaller. In other words the gravity gets weaker in their experience as you near the center.
SO I wonder in our 3D world, if dark matter could simply be a large mass just dimensionally removed enough to be outside our awareness. Like the lead bowling ball. We look at the motion of stars in another galaxy, and we see they are affected by gravity, but cannot see what is causing it.