They say nothing is certain except death and taxes. Make that just taxes.

Osama bin Laden is dead, President Obama told us Sunday.

And in the silence after the initial wave of relief and euphoria, you could just hear the fluttering wings of a host of conspiracy theorists descending.

Now, in the news conferences, we are tearing the details apart limb from limb. The narrative of the capture is already changing — forget the wife who acted as a human shield, enter the shot to the side of the head.

And let the conspiracy theories begin.

In case we needed a reminder of the hum of conspiracy surrounding Sept. 11 itself, there were the tweets from football player Rashard Mendenhall, including a musing about how “We’ll never know what really happened. I just have a hard time believing a plane could take a skyscraper down demolition style.”

Forget martyrdom. The real way to live forever is in conspiracy theory. The major difference between cockroaches and conspiracy theorists is that there is something you can spray on cockroaches to stop them. I don’t mean to be harsh to cockroaches, but their sheer tenacious tendency to crawl out of the woodwork when you don’t want them is only matched by that of people who insist that We Can Never Know What Really Happened.

There are, of course, millions of acceptable reasons that we can never know what really happened — what if everything is an illusion and nothing exists? (“In that case,” as Woody Allen says, “I definitely overpaid for my carpet.”) But conspiracy theorists are unmoved by these philosophical questions. They are confident that everything exists, except of course the moon landing and the JFK assassination As History Records It. And don’t forget the alien autopsies! Of those, they are certain.

Authorities say they are 99.9 percent certain that they killed Osama. What about that 1/10 of 1 percent?

There’s a furious tenacity to that tiny proportion.

Who’s buried in Grant’s Tomb? Why is the Denver Airport shaped like that? Is everything secretly controlled by the Masons? If not, why does our currency look like an explosion in a symbology textbook? The Matrix was a documentary!

First they thought Osama had been dead for years. Now, who knows?! Maybe he’s actually Paul McCartney. Maybe he’s lurking just outside the frame of the Abbey Road album photo, holding Paul’s shoes and scaring him out of step.

Now we’re picking through the evidence, the Sept. 11 families are demanding photos, and even if they release the (reportedly graphic) photos, some people will never be fully satisfied. Doubting Thomas had nothing on us!

Historically, this is the sort of event that attracts conspiracy theories the way that certain women attract live cats and hipsters attract stray paisley scarves.

But it’s getting worse. It used to be that there was a clear mainstream. There was The News, in newspapers, radio, and on broadcast television. Yes, there was still the murmuring, but isolated. The cockroaches had no way of communicating with each other from their hideouts under the davenports of the world. Now that’s no longer the case.

True, the mainstream may not be dwindling as fast as we thought. Social media has multiplied the number of voices, but the number of trusted voices has remained startlingly constant. In general, people are sensible enough to sort the rot from the not. But there are exceptions, and now they have entire Web sites and news networks to themselves.

Instead of these cockroaches’ being dispelled by the spread of media and 24-hour news cycle, they’ve become more visible and more entrenched. It’s the logical tendency of a system where you can drift through your life never having to hear from people who disagree with you. It’s the same impulse that fueled Birtherism.

H. L. Mencken wrote of conspiracists that “The central belief of every moron is that he is the victim of a mysterious conspiracy against his common rights and true deserts. He ascribes all his failure to get on in the world, all of his congenital incapacity and damfoolishness, to the machinations of werewolves assembled in Wall Street, or some other such den of infamy.” It’s a comforting thought — and encouraging it, as provocateurs like Glenn Beck discovered — can goose ratings.

And as long as they do — look at everyone chatting about Mr. Mendenhall’s comments — these conspiracy theorists aren’t going anywhere.

If, heaven forfend, a giant asteroid should smash into the Earth and wipe out all forms of intelligent life, there will still be a cockroach, three fruitcakes and a conspiracy theorist who claims we can never truly know what happened.