my probability that something like string theory is true would go way up after the detection of a Higgs boson, as would my current moderate credence in dark matter and dark energy

One of these things is not like the others, one of these things doesn't belong.

Lots of physicists will acknowledge that string ‘theory’ -- sorry, I cannot bring myself to calling it a theory without scare quotes -- is highly speculative. With 10^500 free parameters, it has no more predictive power than “The woman down the street is a witch; she did it.”

On the other hand, there's no way of explaining current cosmological observations without recurring to dark matter and dark energy (what the hell was wrong with calling it “cosmological constant”, BTW? who renamed it, and why?), short of replacing GR with something much less elegant (read: “higher Kolgomorov complexity”). Seriously, for things measured as 0.222±0.026 and 0.734±0.029 to not actually exist we would have to be missing something big.