I guess it depends on which version of the Force you're talking about. Han's comment makes sense when the Force was just believing in yourself, not using horrible technological devices like targeting computers but awesome technological devices like lightsabers, and tapping into an intuition database to do impressive but still possible-through-mundane-explanations feats. Then, yeah, on that basis, it would not only be possible to not have heard of it but to not accept that it's real.

In the sequels, once it becomes levitation, telekinesis, etc., it becomes a bit harder to rationalize it if you've happened to have seen it, but still possible to not necessarily have been introduced it to it at all.

But in the prequels, once it's all that plus high-profile war and extermination... yeah, that's a bit difficult to rationalize.