I don't understand why my apt-cache would be empty, as there is a gazillion packages installed on my system (like all Ubuntu systems). I tried installing a random program and running it thinking that it might just show temporary packages, but got the same result.
apt-cache show (or
apt show) command needs an argument: the name of a package:
$ apt-cache show gimp Package: gimp Priority: optional Section: universe/graphics Installed-Size: 16188 Maintainer: Ubuntu Developers <firstname.lastname@example.org> Original-Maintainer: Ari Pollak <email@example.com> Architecture: amd64 Version: 2.8.18-1
... and many more lines
Perhaps you want
apt list to see available packages or
apt list --installed for installed packages?