Apple's App Store is touting some impressive accomplishments to mark its sixth birthday on Thursday. For starters, there have been 75 billion downloads to date and developers have earned $15 billion for their creations.

But according to a report from mobile analytics company Adjust about the iOS app life cycle, nearly 80% of the 1.2 million apps currently available are just sitting in the App Store with hardly any downloads at all.

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Adjust uses the term “zombie app” to describe an app that doesn't get measurable attention to regularly receive rankings in the top lists on the App Store. In June, 953,387 apps out of the 1,197,087 available ones (or 80%) were zombie apps, up from 75% in December 2013 and 70% in June 2013.

These percentages may seem high, but the low number of downloads is driven by the fact that app discovery is still a major issue for developers. With 60,000 apps added to the App Store every month, it's becoming increasingly more difficult for apps to stand out. According to Adjust, only one-fifth of apps were sufficiently visible in June.

"The line between living and zombie apps was set conservatively — an app had to rank on any of the 39,171 App Store top lists on two out of three days over the month," the company wrote in a statement.

Apple has more than 39,000 ranking lists to monitor popularity among different categories. It previously ranked the top 350 apps, but recently updated that list to highlight just the top 150, as people weren't looking beyond that ranking number. This has made it even harder for developers to rank.

The report also found that 21% of software that entered the App Store over the years are now “dead” apps that no longer exist, either because they violated App Store terms and conditions or because they were removed by the developers themselves (like Flappy Bird). The categories with the most "dead" (or pulled) apps are books (27%), entertainment (25%) and utilities (25%). Meanwhile, games are twice as likely to be pulled as apps in the general software category, the report found.