Jumping to conclusions is a phenomenon where people reach a conclusion prematurely, on the basis of insufficient information. For example, a person jumping to conclusions might assume that someone they just met is angry at them, simply because that person wasn't smiling at them while they talked, even though there are many alternative explanations for that behavior.
The argument from incredulity is a logical fallacy where someone assumes that if they can't believe that a certain concept is true, then it must be false, and vice versa. For example, if someone doesn't understand how evolution works, they might use the argument from incredulity in order to claim that the theory of evolution must be incorrect, since they are unable to see how it could possibly be true.
... and to bring this up as part of an argument. The issue with doing so occurs when this incredulity isn’t justified or supported by concrete information, and when this lack of belief is used in order to assume that a preferred personal explanation must be the right one, despite the lack of proof.
At the same time, it’s also important to remember that it’s possible that the person using the argument from incredulity is right, despite the fact that their reasoning is flawed.
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.
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.
If possible, you should show that there is scientific evidence that can be used in order to explain the phenomenon that’s being discussed.
In a perfectly rational world, people who encounter evidence which challenges their beliefs would first evaluate this evidence, and then adjust their beliefs accordingly. However, in reality this is seldom the case. Instead, when people encounter evidence that should cause them to doubt their beliefs, they often reject this evidence, and strengthen their support for their original stance.
If you’re trying to explain to someone the issues with their stance, you can mitigate the backfire effect by presenting new information in a way that encourages the other person to consider and internalize that information, instead of rejecting it outright.