A conspiracy theory (e.g., LBJ was behind the assassination of JFK) is often dismissed because, it is said, no party to the conspiracy has ever admitted that there was such a conspiracy or has left any evidence of it.
Why would a party to a conspiracy talk about it or leave any evidence of it? The darker the conspiracy (e.g., the elimination of a president) the less likely it is that a party to it would say anything about it or leave any evidence of it. What evidence that might be found is readily dismissed as circumstantial or coincidental. A person who might have been privy to a particular aspect of the conspiracy, and who years later admits some knowledge of it, is readily dismissed as delusional, senile, or publicity-seeking.
What about the conspiracy against Trump: to defeat him in 2016, to hamstring his presidency, to deny him re-election in 2020, and to legally harass him and thereby make him unelectable in 2024? I would call it an ad hoc conspiracy; it developed over time as various parties acted to advance particular interests of their. But it wasn’t a conspiracy that was carefully planned at the outset and executed according to that plan.
An effective conspiracy leaves no trace but the outcome of the conspiracy. An effective conspiracy is planned and executed by persons who are committed fully to its success and competent to make it succeed without detection of the conspiracy and its workings. Suspicions may abound, but they are left without sound footing.
Absence of evidence isn’t evidence of absence.