The subatomic particle that has almost no mass is the neutrino.
Neutrinos are incredibly light particles that are produced in various nuclear reactions, such as those occurring in the sun or during supernova explosions. These particles are a type of lepton, similar to electrons, but unlike electrons, they have an extremely small mass that is often considered negligible compared to other subatomic particles like protons and neutrons.
Neutrinos interact very weakly with matter, which is why they can pass through planets and stars without being affected. This characteristic makes them challenging to detect and study, but their existence is crucial for understanding fundamental processes in astrophysics and particle physics.