What complicates the calculation derived immediately after determining the Milky Way's total mass from gravitational observations?

Answer

A large portion of the mass is invisible dark matter.

The total mass derived from gravitational effects includes invisible dark matter, often comprising the majority of the mass, which requires subsequent steps to isolate the mass contribution from visible matter like stars.

What complicates the calculation derived immediately after determining the Milky Way's total mass from gravitational observations?
measurementSpaceastronomymethodsstarsastrophysicstelescopesstar countingcounting