To quote u/JackieDaytona74:

Now that being said, the dark ages encompasses around 500AD to 1000AD. The Vatican has many, many, many documents and artifacts that are far older that would provide new insights to our past, Christianity and God. Almost everything the Gnostics wrote was arbitrarily removed by the church (as an example). Even as recently as 2018 Pope Francis was railing against the "threat" of Gnosticism.

And the Gnostic texts are just one singular example out of thousands that the Vatican has hidden. We've been presented a very carefully curated view of Christianity and human history by the Vatican. Heck, even the gospel of Thomas is something the Vatican would rather most people weren't aware of. Despite the fact that the Gospel of Thomas is one of the most important surviving texts for understanding early Christianity. But again, there's a lot of Gnosticism in the Gospel of Thomas so no surprise that the Vatican mostly ignores it.