[gdubs]

No evidence sounds definitive. "We _lack_ evidence" would be clearer, for instance, because it at least suggests that their _could_ be evidence, but it hasn't been found yet.

[RyEgswuCsn]

__lack__ evidence sounds like you have already made up you mind but is just searching for confirmation.

I'd say a better expression would be "we don't know yet whether ...", e.g.: "we don't know yet whether Covid is airborne".

[mrguyorama]

No, actually, definitive would be "We have evidence to the contrary".