This is a frustrating topic to discuss since on one hand there's not much conclusive evidence, but plenty of suggestive circumstantial evidence.

IF this is real, then the evidence is likely held by the governments and therefore any progress in public understanding would be driven by politics rather than science.

What makes me skeptical is how this would be kept secret world-wide. Ok, the US government conceivably could, but there are likely many countries which should have strong evidence in their hands, and it seems unlikely that they all could successfully coordinate on this for so long, across regime changes etc.