Researchers from the Universidad Carlos III de Madrid and the King s College London analyzed a vast data set of 4.4 million malware samples collected between 2007 to 2018 to quantify the amount of Monero (XMR) coins criminals using crypto-mining malware have been able to accumulate. Criminals have mined (at least) 4.3% of the moneros in circulation, earning up to 56 million USD. One of the main reasons of the success of this criminal business is its relatively low cost and high return of investment.
Source: https://www.bleepingcomputer.com/news/security/criminals-grabbed-at-least-43-percent-of-all-monero-coins-on-the-market/