Monero is an open source Cryptocurrency focused towards Privacy and Anonymity.
After Hardforking from Bitcoin, It's been based on the CryptoNote, aswell as boasting untraceable and non-identifiable transaction providing a much more anonymous service than Bitcoin.
DarkNet users have a very strong fondness for Monero due to the recent AlphaBay bust. Monero was the only wallet to be impossible to hack by the Governments. The ETH and BTC wallet's were traced and seized.
Now, The main issue with Monero is its reputation.. Is all this privacy necessary? Up to you to decide for yourself.
Monero Research Paper: https://getmonero.org/resources/research-lab/