With Avengers: Endgame set to break box-office records – it is predicted to make $1bn in its first week – it seems that the superhero business really is the one to be in. In Hollywood, at least.

But what of the medium in which the superhero originated – the comic book – and the purveyors of the hundreds of comics that are released every month? The high street is not as bulletproof as multiplexes, and comic shops are having a tough time of it.

Dozens of closures have been reported across the UK and US over the last few months – including, in January, the end of St Mark’s Comics, once one of New York’s most venerable institutions. (It even appeared in Sex and the City.) Last year, comics website Bleeding Cool documented how 50 comic shops had closed in the previous year, in both the US and UK. And since June 2018, at least 21 shops in the US and 11 in the UK – including shops in Nottingham, Ramsgate and Tooting – have closed, with others likely going unreported.

While superheroes have never had a higher profile, the gap between cinema and comics has never been wider. The days when you could pick the latest issue of Spider-Man or Batman from the newsagent’s shelves are long gone. Last week, comic writer Ron Marz tweeted that, during a presentation to a school class, one girl raised her hand and asked him where she could actually buy comics.

Superheroes have never been more in the public eye, but people don’t emerge from the latest Marvel movie and head to the comic shop Lisa Wood, comic artist

So why are so many going out of business? Like other retailers on the high street, comic shops must factor in rents, business rates, staff wages, insurance – but the profit margins on comics are so narrow as to make this a very delicate balancing act.

“There isn’t a huge profit in comics and graphic novels,” says Jared Myland of OK Comics in Leeds. “Nobody gets into comic retail to be a millionaire. We do it because we love comics. Unfortunately, closures are a more and more common topic on both sides of the pond. Most comic shops make enough to get by if they make cuts, but some retailers depend on the generosity of family and friends to help support the shop.”

One of the unique challenges in comics is the monthly gamble on what will sell. Comics released every few weeks, as opposed to the collected editions available in bookshops or on Amazon, aren’t returnable; with 600 to 1,000 such items published every month, stores must make educated guesses or be stuck with their mistakes. At OK Comics, 90% of what Myland gets in are pre-orders, with the rest put on shelves for casual customers. “Smart retailers would rather under- than over-order,” he says.

Despite the launch of comic-book apps such as the Amazon-owned Comixology or Marvel’s Unlimited, comics have weathered digital challenges better than other print media, according to Rich Johnston of Bleeding Cool, with digital purchases only accounting for around 15% of overall sales. Despite regularly reporting shop closures, he’s upbeat about their future: “Since the 90s there has been a decline, but I think we run as many stories about shops opening as we do closing.”

The old image of comic shops as cliquish, unfriendly places where the uninitiated aren’t made welcome – or, as Johnston puts it, “the sort of den in a basement staffed by a bloke in his 50s with a waistline the same size” – is also fading. Survivors have recognised that they must diversify, by adding board games and food to the mix, while many of the closed businesses end up online. Mike Holman, sales and marketing manager at giant comic distributor Diamond, says that though the industry has changed a lot, “sales are as strong as they have been for 20 years” due to the internet, with 6m comics leaving their hub in Runcorn, Cheshire, every week.

But even those on the inside are worried about the future. Lisa Wood, an artist for the likes of Marvel and DC under the name Tula Lotay, is also a director of the Travelling Man comic shops, after first working in its Leeds branch in the 90s. She says being unable to return comics leads to less risk-taking on unknown names: “Marvel and DC stuff will always do well but for a retailer, when money is tight, taking a risk on new work by unknown creators can end up being very costly.”

Wood adds: “Superheroes have never been more in the public eye, but people don’t come out of the cinema after watching the latest Marvel movie and head to the comic shop. They’ll go online and buy graphic novels at prices the shops can’t compete with. I think that in the future, the industry will move away completely from monthly comics, and just produce graphic novels. But given that the monthly comic market is so important to the survival of physical retailers, it’s worrying.”

• This article was amended on 26 April 2019, the remove a picture of Mega-City Comics in Camden Town, London as an example of a comic book shop. It is, however, not closing.