The intruder installs a rootkit on a computer using a user action or by exploiting a known vulnerability or cracking a password. The rootkit installs a backdoor giving the hacker a full control of the computer. It hides their files, registry keys, and process names, and network connections from your eyes.
Your antivirus could not detect such programs because they use compression and encryption of its files.
UnHackMe allows you to detect and remove Rootkits.