 Find missing hits in IP ranges

All IP ranges have some holes in them for which we don't have a domain name.

It is because there was nothing there, or just because we don't have a good enough reverse IP database?

It is possible that DomainTools could help with a more complete database, but its access is extremely expensive and out of reach at the moment.

Censys is another option that would be good to try.

Putting 140 USD into WhoisXMLAPI to get all whois histories of interest for possible reverse searches would also be of interest.