What is "uncertain" in quantum physics?

Why shouldn't we think of the Uncertainty Principle as just a statement
about the limits of observation, rather than about something objective,
especially as in causing some non-zero vacuum energy?
Is there some experiment that settles "uncertainty" as something "really
there"?  In particular, I am not sure if the expansion of the Universe
is such evidence, or rather a consequence of the theory.
Thanks for any insight.
-Julio