Do you know which OS is used by hackers, and cyber security experts, 

Firstly need to understand what is Linux

Linux is an operating system just like Microsoft, MAC, and Android operating systems.

Pro Linux users do anything with Linux Terminal Coding, create websites, downloading and etc.

Linux have niche os for a specific user: Debian, Ubuntu, Kali, Mint, Arch, Manjaro, Redhat Distributions

Debian is used for web hosting or website. Ubuntu is used for common purposes Just like MAC Book PC.

Most Hackers, Pentester, Bug hunters, and Cyber Security Exper Use Kali Linux Operating System for Testing or Ethical Hacking.