Apparently it thought is was his students trying to pull a prank on him when he got the call
I guess it has taken the Nobels 10-20 years to get their head around the/his idea that we don't know what most of the Universe is made up of.
Actually, the Nobel Prize in Physics 2011
was awarded "for the discovery of the accelerating expansion of the Universe through observations of distant supernovae" with one half to Saul Perlmutter and the other half jointly to Brian P. Schmidt and Adam G. Riess.
It was not awarded because of the "idea that we don't know what most of the Universe is made up of" as you write above.
Perhaps one day in the future physicists will win another prize for the discovery of what most of the universe is made up of. Right now, resolving that problem (what is dark energy and dark matter) remains one of the most important goals of the scientific community.
PS. Brian Schmidt is also American (born in the Montana, 1967), so technically it could be said three Americans won the Nobel Prize.