In the aftermath of the massacre of 26 people in a small-town Texas church, you might have seen that the killer used a gun called an AR-15. It’s a popular weapon—relatively easy to use, endlessly customizable, military in appearance. How popular? It’s the same gun that a killer used in the massacre of 58 people at a Las Vegas concert last month, and by the killer who murdered 49 people in a nightclub in Orlando, and the one at an elementary school in Newtown, Connecticut. And the movie theater in Aurora, Colorado. And the party in San Bernardino, California.

Oh, but wait: It’s also the gun, apparently, that someone in Texas used to shoot back at the killer at First Baptist Church, accurately enough to pinpoint places his tactical vest didn’t protect. “We keep hearing that AR’s are useless for self-defense, that they’re simply ‘weapons of war,’ useful only for mass killing. This is simply not true,” writes David French at The National Review. He didn’t save lives inside the church, French goes on to say, but this straight-from-the-gun-advocate-storybook good guy with a gun “did stop the shooter and prevented him from harming anyone else. He did so with exactly the kind of weapon that the gun control lobby would like to deny to law-abiding Americans.”

Well, OK. Good question then. Is it possible that the AR-15 isn’t just an overpowered long gun beloved by the National Rifle Association but a necessary component of civilian defense in the absence of armed authorities? Somebody should figure this out, right?

Except you can’t. The government doesn’t keep track of how many AR-15s are out there or who owns them. Only through painstaking excavation of crime reports could anyone even begin to figure out which crimes involved AR-15s or when AR-15s stopped crimes—much less where those ARs came from, how they were stored, or how they were modified.

That data is either off-limits or simply doesn’t exist. “If we had easy access to the kind of data we have on motor vehicle crashes for firearm violence, we would be able to answer much more clearly a whole host of questions about gun policy—about which state laws are working, which storage practices work, which guns are riskier than others, what ammunition sizes and magazines matter. I could go on and on,” says Jon Vernick, codirector of the Johns Hopkins Center for Gun Policy and Research. “We’re already in a world where science is questioned, in particular the role of science in policymaking. No one who’s realistic thinks science is sufficient for good policymaking. But it’s clearly necessary.”

You see that lack—or rather don’t see it, I guess—with guns. It comes up with every mass shooting and more rarely when people talk about the epidemic of suicides and accidental gun deaths in the United States. But that data void is growing like the ozone hole in the 1980s, an encroaching Big Nothing. Washington Post politics reporter Philip Bump has been updating a list of things President Trump has undone in office, and an eye-popping number are numbers: oil and gas company payments to foreign governments, corporate salaries organized by race and gender, employer records of workplace injuries, government contractor labor law violations, health effects of mountaintop-removal mining, safety issues at chemical plants, visitors to the White House. Did you want to know any of those things? You cannot.

No one who’s realistic thinks science is sufficient for good policymaking. But it’s clearly necessary. Jon Vernick, Johns Hopkins Center for Gun Policy and Research

Would you like detailed information about arrests, homicides, and gang murders in 2016? Well, the FBI isn’t giving it to you anymore. How about melting Arctic ice? Nope; Congress is dismantling a satellite that was supposed to update the aging monitor network. Climate change? Scott Pruitt, head of the Environmental Protection Agency, doesn’t think human beings cause it and, more importantly, doesn’t really think you can measure anything to find out. The weather? Forget it; the National Weather Service is coming apart at the seams. How many people live in the United States, data critical to determining political representation and funding priorities? Yeah, no—the 2020 Census is shaping up to be an epic disaster.