943. Why do neutrinos only spin left? — BA, Fairbury, IL
The absence of right-handed neutrinos is simply a feature of our universe and I don't believe anyone has a good explanation for their absence. Actually, it's still possible that right-handed neutrinos exist, but if they do exist, then they don't interact with other matter by way of any known force other than gravity. Even left-handed neutrinos barely interact with matter—they experience only gravity and the weak force, and usually pass through the entire earth without being absorbed. It could be that right-handed neutrinos are also present but that they don't even experience the weak force.