Because you can literally see the remnants of the big bang. Basically, if you look out far enough, you see a wall of plasma from a time where the universe was s

Because you can literally see the remnants of the big bang. Basically, if you look out far enough, you see a wall of plasma from a time where the universe was so hot and dense that light couldn't travel freely. It's called the cosmic microwave background and is impossible to explain in a static state universe. Before you get to the wall, you first see younger and younger stars (which we can tell by their composition, which influences their spectra), then a region which is void of galaxies (because it is such a young region of the universe that galaxies hadn't yet formed), and then said wall. Not only that, but the CMB's properties, like its frequency spectrum or its tiny anisotropies match what you would expect to an astonishing degree. https://en.wikipedia.org/wiki/Cosmic_microwave_background https://en.wikipedia.org/wiki/Cosmic_microwave_background