Conduct a site-specific search in Google using the URL pattern site:data.*.gov
Example: The Google search "hypersonic site:data.*.gov" returned 218 U.S. Government data sites with the keyword "hypersonic" on October 19, 2021.
While much of this data may be found through Data.gov, you can search these agency catalogs directly.
Substantially adopted, with some additions, from Table 14.1 in the book Mastering United States Government Information: Sources and Services, by Christopher C. Brown.