From what i understand there are two methods (well as far as i know) to determine how much mass there is in a distant galaxy.
First, Luminosity. By measuring the amount of light produced from a galaxy we can give a rough approximation of how many stars are contained within it. This doesn't include the idea of dark matter.
Secondly, although i'm a bit stumped on how the mathematics work on this, the rotation of the galaxy. I think it has something to do with the coriolis effect. The number of rotations of the galaxy allows us to calculate gravitational influences on the center? of the galaxy. Thus gives us an answer that gives us roughly 1/6 of the amount of mass of the actual result. The other 5/6 of the mass is given through dark matter.
I could be wrong, if so correct me please :)
My idea is, and it's not fully developed yet so bare with me. Multidimensional influences. For example, if you were in a 3 dimensional world with a theoretical 2 dimensional object in it. Given the correct orientation that object would be practically non-existent. But the mass is still there? I only say this because if i were in a 2 dimensional world for something to exist there must be some form of mass, unless of course the laws of physics don't carry over... Which this conversation is somewhat pointless. There's nothing to work off.
So i guess my question is, is multidimensional theories able to give answers to dark matter/dark energy? I guess it works in mathematics, why not real life?
Also, if you have a theory that you think would be better, can you explain it?