The dark ages is a bit of a misnomer. Human culture was in general moving forward and there were enormous scientific advances, especially in the Islamic world. The main reason for the term is that the Christian Church during that time put the clamps down on science, labeling the majority of scientific experimentation as heretical. Scientific knowledge in Britain and Europe basically ground to a halt.

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.

EDIT But you're totally right. There's a lot of non-religious history missing from the Dark Ages and prior and the Vatican has had a long history of gathering and suppressing that data as well.