[retracted]
https://thespacewriter.com/wp/2019/11/16/scientists-are-not-....
Start: "Every week, I see headlines in the mainstream media (as well as the “social” and online media outlets) that say something like “NASA Scientists Baffled at….” or “Scientists Bewildered by…”. It’s annoying and tells me that the writer and/or the headline writer is a) lazy and b) doesn’t have a clue about science or scientists. "
Or is it just a way of talking and you don't want imply that there are non-mainstream media worth reading?
As far as what mainstream media is - I would think that is also obvious. Mainstream media tends to have a broad focus, and not to have analysis suited to experts in the field they report on.
So if one is discussing science reporting and says Mainstream Media it means publications that do not limit themselves to publishing on matters of science, probably don't publish on science very often, and probably don't have staff that know much about science.
Just as when you talk about programming and say mainstream media you are not talking about Dr. Dobbs or some ACM Journal.
As a matter of fact ACM Journals, being focused on relatively narrow subject matters, are not mainstream media.
There is of course mainstream media used as a general pejorative where it is assumed that it exists to disseminate some form of official propaganda that people knowledgeable about some conspiracy can see through.