How the check works
The checker asks the public registration data service (RDAP) about each domain. When the registry has no record, the name has not been registered. When it returns a record, someone owns it.
No registration found is a strong sign, not a guarantee: some names are reserved by the registry or sold at premium prices. Your registrar has the final word.
Picking a startup domain
A short .com is still the default people type. .io, .ai, .app and .dev are common for software, and .app and .dev require HTTPS. Say the name out loud: if people cannot spell it after hearing it, the domain will leak visitors.
Questions
Does checking a domain make someone register it?
This checker only reads the public registry; it does not go through a registrar or record your searches for sale.
Why does an ending say could not check?
Some registries do not publish RDAP data, or answer slowly. Try again, or check with a registrar.
Is a .ai domain a good choice?
It is popular for AI products and usually costs more than .com. It belongs to Anguilla's registry, which sets its own prices and rules.
What if the .com is taken?
Try adding a short word (get, use, try), a different ending, or a new name. Check that the taken .com is not a competitor in your space.