Which other discussion formats? Can you reference or link any? It just wasn't a thing in the public consciousness until very late in 2015, it's ludicrous to suggest otherwise.
I mean it was national news when Andrea Constand originally accused him back in 2005. After Constand came forward other women did as well. They were all old stories with no evidence to support them, but they got attention. One of the early Cosby accusers was interviewed by Matt Lauer on NBC "Today" in February of 2005. Another was interviewed by the Philadelphia Daily News (Beth Ferrier) and that was published in that paper back at the time. Her and yet another accuser had a big article in People magazine a month later.
There's a difference in my mind between "unknown", "viral", and "known to some people." The Cosby shit was the latter-it was known to some people. In the mid-2000s shit went "viral" (which we didn't really even use that word back then) in a very different way than it happens today, it was mostly media driven back then. A story hit some certain threshold of interest/coverage, and suddenly it was wall to wall on the three networks and the cable news networks 24/7. Remember the Elian Gonzalez shit? That was a viral news story in the "old media" era, what was mostly an uninteresting custody dispute got magnified by the Cuban American community, and then the media ran hard with it. To the point there were like 24/7 tracking of the house Elian was living in down in Florida on CNN and shit like that. The Cosby story wasn't one of those stories. The media featured it some, people didn't seem to get that excited about it, and most people seemed to like Bill Cosby. It helped that there was no hard evidence against him. The story died down.
But it still got national coverage and ink, so the idea that it was a secret is...questionable. The whole premise of Hannibal Burress's set was "Google 'Bill Cosby rape' and see all that's out there, it has more results than Hannibal Buress!" This was driven because Cosby had taken to shitting on the younger black comedians, basically saying they needed to stop cursing, pull their pants up etc, so Hannibal was throwing one back at Cos. For whatever reason, it stuck and went viral. But it was actually predicated on the fact all this information was out there, it wasn't secret.
It can get dicey when we start assuming what is "common knowledge." For example to us gaming nerds, this Blizzard drama is common knowledge. If I called my Mom up and said "hey did you hear about all the sexual harassment shit at Blizzard?" She'd be like "Blizzard? What are you talking about? Is it snowing?" I've always been a news junky, I used to play EQ in High School with the news on TV in the background. I've always skimmed most of the major papers every day, and by the mid-2000s I had a big RSS News feed I'd scroll through a few times a day. Not everyone is like that, but most people are part of some "knowledge group" where knowledge of something is very common but isn't that common to outsiders. With the Cosby stuff, where it had been on literal national network TV, I think it's safe to say it was public knowledge. It just wasn't "pervasive" public knowledge. Bill Clinton getting a blow job was pervasive public knowledge. Kelsey Grammar being a coke head was something that had been in the news, but didn't seem to affect the popularity of Frasier or his public standing. Most people if you asked them today would probably say they didn't know Grammar was a drug addict, but at the height of his career it was publicly known and posted in papers and etc.
Edit: I completely forgot to link this specifically in response to your question, the article is paywalled but worth reading if you can get by it--it's a Washington Post article from 2014 basically asking "we knew 8 years ago about these Cosby allegations, why is it just now becoming viral?" It details the national news coverage of Cosby's allegations in the mid-2000s, and speculates on why people just collectively shrugged it off:
https://www.washingtonpost.com/life...d57d00-6dd0-11e4-8808-afaa1e3a33ef_story.html