One example of something that can travel at the speed of light but not be seen by humans is a theoretical particle called a neutrino. Neutrinos are subatomic particles that have a very small mass and no electric charge. They interact with matter extremely weakly, which means they can pass through ordinary matter, including our bodies, without being detected.
Neutrinos are produced in various natural processes, such as during nuclear reactions in the Sun or in supernovae explosions. They are also created in particle accelerators and nuclear reactors. Because they interact so weakly with matter, they can travel long distances through space or even through the Earth without being significantly absorbed or scattered.
Although neutrinos can travel at the speed of light, they are invisible to the human eye because they rarely interact with photons, which are the particles of light. Special detectors called neutrino detectors are used to indirectly detect neutrinos by observing the products of their interactions with other particles. These detectors are typically large and sensitive instruments that can detect the faint signals left by the rare neutrino interactions that do occur.
So, while neutrinos can travel at the speed of light, they remain invisible to humans because they interact so weakly with matter, including the particles of light that we rely on for vision.