Note: You are currently viewing documentation for Moodle 3.1. Up-to-date documentation for the latest stable version of Moodle is probably available here: admin/environment/php extension/dom.
Certain Linux distributions do not have this extension included in the minimum PHP package. It can usually be found in one of the optional php-* packages.
For CentOS, you will need to run yum install php-xml, which provides this extension.