If you can’t find your keys, and you search your entire car, and you don’t find your keys in your car, it’s okay to say your keys are not in your car. You are making an argument based on lack of evidence. This argument is valid because you searched the entire car.
If you argue that aliens do not exist because you have never seen evidence of extraterrestrials, you are also making an argument based on lack of evidence. But this argument is not as strong, because you have not searched the entire universe for evidence of aliens. In fact, you probably have not even searched a significant portion of the universe. So the lack of proof of extraterrestrials does not necessarily prove that extraterrestrials don’t exist. Maybe they do exist; you just haven’t looked in the right places!
The Joy of Argument, “Lack of Proof” pg. 98
These examples illustrate how carefully you must examine lack of proof in an argument. On first glance a lack of proof may indicate that something does not exist. But this is only a valid conclusion if all possible sources of proof have been explored.
Some arguments have a conclusion that provides certainty, while others have a conclusion that only provides a probability. Be careful when arguing about lack of proof. What may at first appear to be a strong defense, could easily end up to be an assumption based on lack of evidence.