On the other hand you can look for eventcodes that a domain controllers generates:
For example 4776: The domain controller attempted to validate the credentials for an account
index=*win* source="*WinEventLog:Security" EventCode=4776
| rename ComputerName as DomainControllerName
| table _time DomainControllerName user
This will give you a list of your domain controllers....as long as you have windows clients sending their eventlogs to Splunk ofcourse.
SPL cannot query a network. SPL queries data stored in Splunk indexers. If you have indexed a list of your servers in Splunk then SPL can be used to query that data to find DCs.
There are exceptions, of course. The Splunk for Asset Discovery app (https://splunkbase.splunk.com/app/662/) uses the nmap utility to scan networks for devices and indexes the results. The Splunk Supporting Add-on for Active Directory app (https://splunkbase.splunk.com/app/1151/) can query Active Directory for information, which might include DCs.
OTOH, if your DCs are reporting events to Splunk now, you can use this query to get their names. Modify the "dc" to match the name scheme for your DCs.
| metadata type=hosts | search host="*dc*"