"Know" in science is used informally to mean "the preponderance of evidence supports this conclusion. Which could turn out to be wrong if enough contrary evidence is later found."

The scientific consensus today is that the evidence supports the idea that not all the dinosaurs died out when the Chicxulub comet struck 66 million years ago. The ones that could fly or quickly learn how to fly survived and even thrived, and their grandkids are in your back yard right now.

https://www.birdlife.org/news/2021/12/21/its-official-birds-...