Superman regretted that he forgot to ask the Collector of Worlds whether anything from Krypton was preserved.
The planet Krypton is the home of Superman and all other Kryptonians. The doomed planet was destroyed by a natural disaster, and Kal-El was thought to be the only survivor. Later, it was revealed that other Kryptonians survived as well, either in the Phantom Zone (a pocket dimension prison) or the bottle city of Kandor (shrunk by Brainiac). The Kandorians eventually rebuilt the planet, in the Silver Age as Rokyn, and in the modern age as New Krypton.