Jesus was a shape shifter

[popeyespappy]

How do you know there's dark energy and/or dark matter, I hear scientist saying it exist though they haven't seen any.

The simplest answer is we don't. The existence of dark matter / energy is a theory that explains our observations. It is a theory supported by evidence such as dark matter's gravitational effects on visible matter, radiation, and the large-scale structure of the universe. That's the difference between us and you. We accept it as the best explanation for what we see. We don't accept the existence of dark matter / energy on faith alone. Nor is our acceptance static. If further observations produce new evidence that points to an explanation other than dark matter / energy then we are OK with that.