Here is a mainstream ''conspiracy theory'' :

SARS-2 / CoViD-2019 leaked from the Wuhan Institute of Virology, in China, and it was engineered as part of a Gain of Function experiment, likely partly-funded by the NIH, as that US agency admited to funding GoF research on bat coronaviruses ( but totally not THAT one, lol ).

This was highly suspected, then confirmed, but higher-ups decided to lie and cover it up in an actual conspiracy, as the Fauci e-mails proved. They knew of the research conducted at the WIV, they knew that in Fall 2019, a bunch of researchers at WIV got sick before the ''wet market'' epidemic started.

The researchers who published a letter in Nature swearing SARS-2 being a GoF engineered virus was ''scientifically impossible'' had ties to the Wuhan Institute of Virology, a conflict of interest they ''forgot'' to disclose. They knowingly lied and decieved the public, and these lies were uncritically published by one of the most proeminent scientific journal of the entire world.*

So yeah, a ''mainstream conspiracy theory'', also known as ''verifiable fact''.

(* Even if virologists weren't directly tied to the WIV, it is in their selfish, partisan interest to deny their collegue's experiments caused the spread of a new virus all over the planet, because a backlash against that would get their funding cut.

For months they swore they were not doing GoF, then when they got exposed in comission pivoted to saying ''yes, we do it, but it's akshually a good thing because we need to research the bioweapons we make to inject you ''vaccines'' against those bioweapons, trust the Science bro''. )