Besides the obvious stars and planets, you might think that the surrounding space is completely empty — a perfect vacuum devoid of all matter. (businessinsider.com)
Hsu realised that running the process backwards would be equivalent to a black hole forming and then existing in a perfect vacuum, with no Hawking radiation. (newscientist.com)
Strangely enough, the simplest example of energy with repulsive gravity involves Einstein's nemesis, quantum mechanics, which holds that even a perfect vacuum is not empty. (discovermagazine.com)