That's not how it works. That's where the difference in scientific and legal baseline comes into play. If I claim I saw a shark with 8 legs walking across the neighborhood and my friend refuted that claim, the burden of proof would be up to me a common knowledge and what we know would say the baseline is that it doesn't exist. If you said there were rivers on Mars 100,000 years ago and I said there werent, the burden of proof would lie in whoever made the claim, for or against. In this case the baseline would be the unknown and the burden of proof is on the prosecutor