The Sun existing is an assumption, if asked to “prove it does” you can’t. That doesn’t mean I won’t burn or anything like that if I stay out too long, but trying to prove that something exists is tricky.
Your examples of morality don’t get around the fact that it is a subjective matter. Right and wrong are not objective and change more often than I would like. You can argue why murder and such are “wrong” but it would ultimately boil down to “because I or someone else says so”.