Neutrinos are tiny particles that are produced by certain types of radioactive decay and other processes. They have very little mass and are electrically neutral, which means they don't have a positive or negative charge.

Anti-neutrinos are also tiny particles that are produced by certain types of radioactive decay and other processes. They also have very little mass and are electrically neutral, just like neutrinos. However, they have a property called "spin" that is opposite to that of neutrinos. This means that they behave in a slightly different way when they interact with other particles.

The idea of "anti-particles" might seem strange at first, but it's actually a very important concept in physics. Every type of particle has a corresponding anti-particle, which has the same mass but the opposite charge. For example, the electron is a type of particle that has a negative charge, and the anti-electron (also known as the positron) has a positive charge.

So, to answer the question, anti-neutrinos can exist because they are a type of particle that is just like a neutrino, but with a different spin. Even though they are electrically neutral and don't have a charge, they can still be distinguished from neutrinos because of their spin.