Extragalactic systems (like the spiral galaxy NGC 7757 pictured at right)
show clear evidence for mass discrepancies. When we measure the amount
of luminous mass (stars and gas) in these systems and apply Newton's
Law of Gravity, the observed mass falls well short
of the amount required to explain the observed orbital speeds.
This is the mass discrepancy.
There are two logical possibilities to explain the observed mass discrepancies:
Either
There are significant challenges to both ideas.
Issues for MOND:
Issues for Dark Matter:
My gut reaction to all these questions is negative.
But it appears that one set or the other must be answered in the affirmative.
Either way, we are missing something fundamental about the nature of our
universe.