It seems to me that once black holes were theorised then the obvious singularity at its centre - though only ratified after Hawking and Penrose's singularity theorems - one would have to ask where the matter and energy went to, if we are to save the conservation of mass-energy. After Einstein & Rosen theorised the Einstein-Rosen Bridge, the otherwise known, wormhole, the obverse of a black hole could be thought: that is a white hole. Here, matter or light can't enter it, but they can escape.
Was this the earliest conceptualisation of such, or was there an earlier conceptualisation? There's very little history about this in the usual sources.
**@Ben Crowell: Although I dreamt up the question, I don't think of it as a 'nice' question as its a little too simple-minded for that. It's no mistake on my part, as I'm referring to the original notion of a black hole by John Michell in the late 19th C though he called them dark stars. And the insight that gravity would produce a singularity was the insight of Roger Boscovitch if there was no countervailing pressure - though for him - this led to the notion of atoms and interatomic forces that push back against gravity in order that matter is stable - and this was well before atoms became fashionable.