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Somebody wrote that headline.
On Google News today, the above headline appears, attributed to the Atlantic magazine. The actual article has a different headline today, and I don’t know whether the above was the original headline, and somebody got to the Atlantic after Google News had reproduced its headline; or somebody connected with Google News came up with it—probably the former. But does anybody really think that “cosmos” is a plural noun? I’ve seen kudos treated as a plural noun several times, but that’s not a very common word, and one can imagine that lots of people have encountered it only in writing, and have no idea that it’s a singular Greek noun. But cosmos is a fairly common word. It’s been the title of at least two books and at least two TV series. There’s really no excuse.
Crutchfield's Orthoglossary 
I did see a headline a while ago that treated “innocence” as a plural noun (i.e., “innocents“), but I assumed that headline was written by a bot.
So it should be “cosmos is?” I’ve not thought about it, at least not since the death of Dr Sagan.
Another possibility, of course, is that the writer, whoever it was, imagined that the subject was people named “Cosmo”, some of whom are really, really big.
Oh! I just noticed that the title of the HTML page is “We May Never Know How Vast the Cosmos Really Are”. Looks like somebody at the Atlantic needs his or her knuckles rapped.
My learned New York brother points out that there is a drink called a “cosmo”, short for “Cosmopolitan”, apparently popular among young women who read or model (or wish to model) for the magazine of that name. Maybe the writer of that headline is such a woman, and the drinks she enjoys are unimaginably vast.
Where have all the editors gone?
Verizon’s web forum, https://forums.verizon.com/, lets you award a post a “kudo”. That’s not all that’s wrong with Verizon.