Argument from ignorance is one of the simpler informal fallacies to learn — in short:

We do not know that X is true — therefore it is false.

We do not know that X is false — therefore it is true.

In cases like this, you have to ascertain how much evidence is needed to make your argument hold water. Not knowing whether or not something is true or false isn’t a good enough case to claim one way or the other. When you’re lacking in evidence, it’s best to start getting some — or just simply admit that you don’t know what the situation really is!

It’s also known as argumentum ad ignorantiam or “negative proof”; you can also keep in mind that “absence of evidence is not evidence of absence”. There’s a requirement of burden of proof from the claimant — and personal belief or argument from personal conviction just isn’t enough proof to make it so.