I'm not going to discuss the specific issue, but, as a generalisation, I would suggest that only a very arrogant person would believe that something definitely did not exist unless they (or even 'anyone') were/was currently aware of convincing evidence of its existence.And please don't try to dismiss the principle of requiring evidence as "an opinion".
I would have no problem with your saying that you were unaware of any evidence for the existence of X, but to jump from that to an assertion that X does not exist is as unscientific as it is to believe in something for which there is no evidence.
You only have to go back relatively small amounts of time to the days when there was no evidence for the existence of all sorts of things which the passage of time has clearly demonstrated do exist.
Kind Regards, John