Why only left-handed neutrinos have been observed yet? This is what I understood (it may be wrong, so please correct me): since every particle is "divided" into a lefthand and a righthand partner, which is what guarantees the particles to have mass, and since it's finally proved that neutrinos have mass, why only lefthand neutrinos have been observed hereto? 
 A: The only way to detect neutrinos is through the weak force as they do not interact through the strong force or electromagnetism and their gravity has too small an effect to be detected and the weak force does not interact with right handed neutrinos rather or not they exist.  Even if right handed neutrinos exist it is unlikely we will ever detect them.
A: According to the Standard Model, the forces which govern particle physics are:


*

*Color Force (strong nuclear force)

*Weak Force (Weak nuclear force)

*Electromagnetism


when we write down the interactions of neutrinos, the standard model predicts that they are invisible to the color and electromagnetic forces. However, they do couple to (interact with) the weak force. The weak force is a bit strange in that it only interacts with the left handed version of particles. Thus, the right handed neutrino is also invisible to the weak force. This makes it so that only left handed neutrinos are detectable since right handed neutrinos (if they exist) don't interact with any of the standard model forces. 
