Generally speaking, when there is no evidence that something exists, it is common practice to say it doesn't exist until there is evidence to the contrary.

There aren't any purple unicorns on this planet. That phrase is acceptable despite the fact that the reality is that I have never seen a purple unicorn but there could be some somewhere (perhaps in the bottom of the ocean?). When there is a lack of evidence of something, you don't generally assume it's out there somewhere, you assume that unless there is some evidence to the contrary, it doesn't exist.