Relativistic particles are particles that move at speeds close to the speed of light, where the effects of Einstein's theory of relativity become significant. As their velocity increases, their mass effectively increases, leading to greater energy requirements for further acceleration. This phenomenon is crucial in high-energy physics and astrophysics. In practical terms, relativistic particles are often studied in particle accelerators, where protons and electrons are accelerated to near-light speeds. Understanding their behavior helps scientists explore fundamental questions about the universe, including the nature of dark matter and the conditions present in black holes.