Former President Barack Obama says one of the biggest threats to our democracy is Americans not agreeing on what’s fact versus opinion.

“One of the biggest challenges we have to our democracy is the degree to which we don’t share a common baseline of facts,” Obama told David Letterman in the first episode of Letterman’s new Netflix series “My Next Guest Needs No Introduction.”

Letterman asked Obama during the interview whether the leader of a democracy diminishing the press or “somebody screwing around with the actual voting process” was more damaging to that democracy.

“What the Russians exploited, but it was already here, is we are operating in completely different information universes,” Obama said. “If you watch Fox News you are living on a different planet than you are if you listen to NPR.”

Obama said at the time of his presidential campaign in 2007 people didn’t realize the power or dangers of social media.

“We essentially built what ended up being the most effective political campaign probably in modern political history. So I had a very optimistic feeling about it,” Obama said of his campaign’s social media use. “I think what we missed was the degree to which people who are in power ... can in fact manipulate that and propagandize."