Unfortuantely, the only way to expand the centos-root VG in this case would be to have a new disk presented to the system.
Then you can add that disk to the VG with vgextend, then increase the root LV with lvextend.
Unless, by chance, you have unused disks already presented to the system.
You can check this by running: fdisk -l |grep sd
Then look for any disks that do not have a partition created. Or you can run pvs to find any disks with a physical volume created that are not part of any VG. Those could also be used.
Additionally, have you checked the size of the log files that Splunk is generating on your machine? Depending on how busy it is, and how long it's been running, those can add up quickly. Especially if you don't have logrotate configured.
Try checking those with these commands (assuming /opt/splunk is your $SPLUNK_HOME):
du -sh /opt/splunk/var/log/splunk/
du -sh /opt/splunk/var/log/introspection
ll -h /opt/splunk/var/log/splunk/
ll -h /opt/splunk/var/log/introspection
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