1. Explain why this sort of reasoning is fallacious: namely the fact that your opponent’s inability to explain a certain phenomenon or to understand a certain theory, does not invalidate current explanations for it.
2. Shift the burden of proof back to your opponent: ask them to support their initial assertion, and explain why they are incredulous, and why they think that this validates their position.
3. If possible, you should show that there is scientific evidence that can be used in order to explain the phenomenon that’s being discussed. 

Is a logical fallacy where someone concludes that since they can’t believe that a certain concept is true, then it must be false and vice versa.

Its 2 basic forms:

“I can’t imagine how X can be true; therefore, X must be false.”

“I can’t imagine how X can be false; therefore, X must be true.”