>Are there things that we can't test that do exist?
There's many reasons to believe that objects that exit our light cone continue to exist after they do, even though they could never have any future interaction with us to confirm that. (Say a spaceship leaves Earth at near the speed of light in a straight line, and then enough time passes that the space between the ship and Earth is expanding so fast that the spaceship or any kind of signal from the spaceship would have to travel faster than light to return to Earth, which is impossible. Believing that the spaceship disappears when it exits our light cone requires believing in unnecessarily more complicated physics.)
There's many reasons to believe that objects that exit our light cone continue to exist after they do, even though they could never have any future interaction with us to confirm that. (Say a spaceship leaves Earth at near the speed of light in a straight line, and then enough time passes that the space between the ship and Earth is expanding so fast that the spaceship or any kind of signal from the spaceship would have to travel faster than light to return to Earth, which is impossible. Believing that the spaceship disappears when it exits our light cone requires believing in unnecessarily more complicated physics.)