During two days testifying before congressional committees in Washington, Facebook CEO Mark Zuckerberg insisted again and again that users of the social network he founded had complete control over what they put into it.

“Every piece of content that you share on Facebook, you own and you have complete control over who sees it and how you share it, and you can remove it at any time,” he said.

But this user-posted content — photos, links, text, likes and other types of information voluntarily made public — comprises just one layer of the data on Facebook. Zuckerberg seemed uncomfortable talking about the other layer.

Reining it in: Congress: No clear ideas about how to regulate Facebook

Besides visible content, there’s another type of data that is not immediately obvious, a kind of “dark matter” data. This is the information Facebook gleans based on your behavior both on and off Facebook, along with what you voluntarily post. It’s this data that makes Facebook feel “creepy” to some, that invokes ads that seem eerily timely.

Facebook uses both user posts and this dark-matter info to build a set of interests associated with you. If you routinely click on country music videos your friends post to Facebook, the site may add “country music” to your list of interests. If you share a lot of posts from progressive political pages and sites, it may tag you as a liberal. If you have listed the town where you are born but are not living there anymore, Facebook may tag you as “away from hometown.”

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During Wednesday’s four-hour grilling by the House Energy and Commerce Committee, Zuckerberg grew increasingly uncomfortable when asked about data collection done away from Facebook’s platform, which adds to the trove of dark matter info on each user.

In fact, the data are collected even from people who don’t have Facebook accounts through an advertising network it operates that publishes ads on other websites.

Hot seat: Zuckerberg faces calls for Facebook regulation in House hearing

If you don’t have an account, Facebook stores information about your activity on websites that participate in its network in what has been dubbed a “shadow account.” Though Zuckerberg said he had not heard of the term, he acknowledged these profiles exist but said they are used to make sure someone doesn’t sign on and impersonate someone else, “for security purposes.”

The public’s discomfort with this type of data collection could ultimately lead to Facebook being regulated by the government.

In addressing the House committee and, a day earlier, the Senate Commerce and Judiciary committee, the 33-year-old billionaire sometimes appeared to have difficulty communicating with the lawmakers questioning him. He never lost his cool and always sought to soothe the occasional outraged feelings.

For example, he explained patiently several times that Facebook doesn’t sell data to advertisers. Rather, it uses the data it has to let advertisers surgically target users who might be interested in their products.

Check it: How to see if Cambridge Analytica got your Facebook data

Had the questioners found a way to articulate the kinds of questions that would elicit more direct responses from Zuckerberg, they might have gotten to the root of Facebook’s creepiness problem.

User can find the interests Facebook stores for them and delete them, although they can be hard to find. On the Facebook web page, go to Settings, then Ads, then Your Information, then Your Categories. Hover over each one to get more information about them, and click the X that appears to remove it.

On Wednesday, one legislator expressed exasperation at the amount of tracking the social network does with its users, both on the platform and off.

“Facebook now has evolved to a place where you are tracking everything,” U.S. Rep. Kathy Castor, D-Fla., said. “It’s practically impossible to remain untracked in America.”

Over a total of nine hours of testimony and questions, it became clear that not just Facebook, but the internet as a whole, is driven by the gathering and monetization of lots of personal data. This has been true for a long time, but with the committee hearings, Americans paying attention perhaps got their first real picture of how it all works.

Get private: How to lock down a Facebook account or just delete it

The catalyst for this two-day marathon was the revelation earlier this year that a British consulting firm called Cambridge Analytica grabbed the personal data of 87 million Facebook users without their consent. The information was later used by first the Ted Cruz and then Donald Trump presidential campaigns ahead of the 2016 election — even though Facebook had changed its policy two years before to prevent such types of collection.

Zuckerberg wasn’t immune from the data gulping. He revealed that he was one of those whose information was snatched up. Apparently one of his friends had used the “Your Digital Life” app created by a researcher, which took in the data of those who installed as well as all their friends. The information was then sold to Cambridge Analytica.

Mysteries: What you don't know about how Facebook uses your data

Several committee members who understand about dark-matter data tried to pin down Zuckerberg on how well users can control this type of collection, and he was not as detailed in his responses as when discussing the more visible layer. To be fair, many of the questioners didn’t understand Facebook well enough to ask questions in a way that Zuckerberg could answer.

U.S. Rep. Ben Lujan, D-N.M., pushed on this issue, and Zuckerberg said that even people who don’t have Facebook accounts can see what data are collected on them and opt out. But Lujan said he was unsuccessful when he tried.

“And it may surprise you that on Facebook’s page when you go to ‘I don’t have a Facebook account and would like to request all my personal data stored by Facebook,’ it takes you to a form that says ‘go to your Facebook page and then on your account settings you can download your data,’” Lujan said.

The congressman didn’t explore far enough. Underneath that box on the page is a button that says, “This didn’t answer my question.” If you click that, a set of checkboxes will pop up and ultimately lead you to a form for requesting or opting out of data.

Myth busting: Facebook isn't listening to you through your smartphone.

During both sessions, members brought up the conspiracy theory that Facebook is listening to people through their cell phones, then using that information to serve ads. People claim they have just been discussing something verbally with a friend, then launch the Facebook page or app to find an ad promoting what they’d just talked about.

Zuckerberg vehemently denied this, as he has before. He told U.S. Rep. Larry Bucshon, R-Ind., that “my understanding is that a lot of these cases are coincidences.” After talking about something, one of the parties searches for the topic or product on Google or even Facebook, causing the ad to appear on the pages of the other party in the conversation.

What Zuckerberg didn’t say, but could have, is that Facebook doesn’t need to listen to you via your smartphone. It already has enough data on you to make it appear that it’s listening. For many, that may be even more disturbing.

dwight.silverman@chron.com

Dwight Silverman is the technology editor for the Houston Chronicle and the grillmaster for the TechBurger tech news site. Follow him on Twitter and Facebook.

Get more tasty tech news at TechBurger. And follow us on Twitter and Facebook.

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