It is nearly impossible to prove a non-existence (e.g., "unicorns don't exist").
In debate or philosophy, the person making a claim is responsible for proving it.
If you find yourself in a situation where the burden of truth matters: Who made the initial statement?
Does the proof match the scale of the claim?
To help you apply this, are you looking at this from a , philosophical , or personal perspective?
"What can be asserted without evidence can be dismissed without evidence."