Even then, a common bit you'll hear from people actually defending pedophilia is that the damage caused is a result of how society reacts to it or the way it's done because of the taboo against it rather than something inherent to the act itself, which would be even harder to do research on than researching pedophilia outside a criminal context already is to begin with. For starters, you'd need to find some culture that openly engaged in adult sex with children in some social context and was willing to be examined to see if the same (or different or any) damages show themselves.
And that's before you get into the question of defining where exactly you draw the age line before it "counts" as child sexual abuse, which doesn't have a single, coherent answer. The US alone has at least three different answers to how old someone has to be before having sex with them is not illegal based on their age alone (16-18, with 16 being most common), with many having exceptions that go lower (one if the partners are close "enough" in age are pretty common). For example in my state, the age of consent is 16 with an exception if the parties are less than 4 years difference in age. For California in comparison if two 17 year olds have sex they've both committed a misdemeanor unless they are married.
Not just on the dark web (which technically is anything not indexed by search engines) but hidden sites are specifically a TOR thing (though Freenet/Hyphanet has something similar but it's called something else). Usually a TOR hidden site has a URL that ends in .onion and the TOR protocol has a structure for routing .onion addresses.