How certain are we of the existence of quarks?

A friend of mine today (who is more often than not full of shit, but I try to give the benefit of the doubt) claimed that he doesn't believe that quarks exist. He says that there is not enough "proof" in their existence for him to say they exist, not that he isn't open to the idea. So, how certain are we that quarks exist? If so, can anyone cite any experiments or other proof that proves their existence and why? Is it possible that they are just a plausible theory that works out mathematically but isn't actually real, like the old theory of the universally permeating "ether" throughout the universe that was used to describe various phenomena, but ultimately was proven to be false? Or are we completely certain of them?