Four centuries ago, when Galileo first trained his telescope on the night sky, light—“the messenger of the stars”—revealed to him and to humanity a multitude of celestial bodies that had never been seen before. From that day onward, increasingly sophisticated instruments have captured entirely new colors and wavelengths invisible to the naked eye, gradually unveiling the hidden wonders of our Universe. The discovery of cosmic rays soon followed, sparking one of the most thrilling intellectual adventures in scientific history. In this seminar, Alessandro De Angelis retraces that extraordinary journey—already forging ahead into the future—and showcases some of today’s cutting-edge telescopes for ultra-high-energy photons, neutrinos, and gravitational waves: the hallmarks of 21st-century astronomy that are opening new frontiers in exploring the hidden Universe.