It isn’t. We just can’t see most of these interstellar billboards. Space is full of light energy on a spectrum from high-energy gamma rays to low-energy radio waves, says Mark Hammergren, an astronomer at Chicago’s Adler Planetarium. Our eyes can see only a small portion of that gamut, which scientists have aptly dubbed “visible light.” Ultrashort gamma waves can’t penetrate Earth’s atmosphere, for example, and our retinas simply can’t register the super-low-energy infrared waves.
