i know i have a heart, lungs and spine in my body. I have never cut myself open and looked for myself, but people trained in such things have looked inside humans and found these organs. time after time these organs are found inside humans. i probably have them too.
i have never been to space, but HUNDREDS of people have. they took pictures, captured video, brought stuff back from space, and left stuff there. if i were to go to space, I'd see a round earth, just like them.