So you are asking someone to prove a negative, which is a logical fallacy, something which people in college will learn about on how to present in a debate/discussion.
To use your example, we also do not know for sure if unicorns exist in this world. The onus to prove that it exists lie with someone who claimed it exists, not on someone who claim it did not exist. Just like in court, if you are accused of murder, you do not have to prove you did not murder, it is the accuser who has to prove you did murder. Get it?