Those making the original assertion need to bring evidence. Without that evidence, those challenging the assertion have no need to bring evidence. For those making the assertation to ask for evidence as to why their conjecture is correct is the Burden of Proof fallacy, which is common when the original conjecture has no basis in principle, fact, or experience - it's just a conjecture. All attempts by the original assertion author to invert this fallacy should be strongly rejected, since it shows a lack of understanding of who ideas are put forth and tested in the larger marketplace of ideas.