Bar shifting is a standard practice in Debunking and State Pseudoscience that consists in arbitrarily changing the standards of evidence required to prove a theory depending on whether it conforms of not to the officially accepted one.
When it comes to alternative theories, the bar is raised to the point that "proving" something becomes virtually impossible: if opponents cannot prove their case with 100% complete explainations under 100% mathematical certainty, all evidence presented is dismissed in block, as if no evidence was presented at all. Since such a level of mathematical perfection is inexistent in science, this effectively guarantees no alternative theory can ever be "proved".
When it comes to accepted theories, the exact opposite strategy is used and the bar is lowered to the point that anything can be "proved" with virtually zero effort; if it is possible to come up with any remotely plausible explaination for a problem, that explaination is automatically treated as the correct one, and the problem is marked as "solved".
In order for this trick to work is important to always treat every problem in isolation, in order to avoid a comparative pro/cons analysis that might show the alternative as clearly superior in light of Ockham's Razor.