Voted for Yes, they must exist, but there was no contact yet.
One explanation for the paradox might be that there is a moral code across the extraterrestrial society that prevents aliens to officially contact us, until we have reached a certain level of development or fail to do so. Some occasionally nevertheless do so, what we then see as UFOs. We are like that yet uncontacted Sentinelese tribe.
https://www.survivalinternational.org/tribes/sentinelese