The Basic Issue
The evidence for mass discrepancies in extragalactic systems is clear.
But what is the explanation?
Either
- Most of the Mass in the Universe is Invisible (Dark Matter), or
- Dynamical Laws must be Modified (MOND).
There are significant challenges to both ideas.
Issues for MOND:
- Can a modified force law explain all observations?
- Can a theory encompassing both General Relativity and MOND be found?
Issues for Dark Matter:
- Does the stuff we call Dark Matter really exist?
- Can a dark matter based theory explain the MONDian phenomenology observed
in rotation curves?
My gut reaction to all these questions is negative.
But it appears that one set or the other must be answered in the affirmative.