Technically, the very first films created could be considered documentaries. In 1878, when Horse in Motion, the first piece of work that could be considered a movie, was created, it was capturing and displaying something that had happened in real life (in fact, that was the point), rather than creating fiction.
Documentary films, since then, have always been a part of the film landscape — even when those films claimed to be true-to-life but were instead fiction, such as the infamous Nanook of the North.
Since the advent of Netflix, Hulu, and other streaming services, however, documentaries seem to be having a moment in the sun. Quoting from an IndieWire article in 2017:
Although documentaries being seen on smaller screens may be a relatively newer phenomena, Kevin Iwashina, head of sales shop Preferred Content, indicated that it isn’t necessarily at the expense of an older viewing habit. “It’s not like there was once a hugely robust theatrical documentary marketplace that was all of a sudden destroyed by Netflix,” he said. “What Netflix and other Streaming Video on Demand platforms have done is to expand the access consumers have to documentary content.”
“There is a sense of optimism about the documentary marketplace,” he said. “There is innovative storytelling; there is prestige; and it’s in the nascent stages of evolving into a tangible business model.”
And yes, the fact that documentary filmmakers aren’t really able to survive with their filmmaking as a tangible business model is a concern. As the International Documentary Association found in 2016, quote:
Only about 22 percent of documentary professionals say they are able to make their primary living from documentary filmmaking; about three-quarters (78%) say they are not at all able to make a living from their documentary work. Combined, more than two-thirds (66%) of documentary makers made either no salary at all (36%), or less than 50 percent of their salary (30%), from their most recent documentaries.