Again, quarks are detectable in a variety of ways. Collision experiments in the mid-60's showed there to be particles inside protons. Quarks are detectable through the properties of the particles they produce. In fact, one of the first predictions of the quark theory was the existence of a particle that was then confirmed. Since then, not just the original three quarks (up, down, strange) have been detected, but three more have been (charm, bottom, top). What we have NOT seen is a *lone* quark. But we have detected many quarks. Furthermore, the quark theory is testable: it makes very specific predictions about the types and properties of subatomic particles that can be tested in accelerators.<quoted text>
Like quarks, eh?
Or your intelligence, eh?
Intelligence is detectable in a variety of ways, including discussions like this.
Once again, to exist means to be detectable and testable, at least in theory.
Let me put it this way: how do you know there isn't an elephant in your room?