We don't even know everything we don't know – a fact that's been made evident in a new discovery. While running equations for quantum gravity corrections for the entropy of a black hole, a pair of physicists found that black holes exert pressure on the space around them.
Not much pressure, to be sure – but it's a finding that's fascinatingly consistent with Stephen Hawking's prediction that black holes emit radiation and therefore not only have a temperature, but slowly shrink over time, in the absence of accretion.
"Our finding that Schwarzschild black holes have a pressure as well as a temperature is even more exciting given that it was a total surprise," said physicist and astronomer Xavier Calmet of the University of Sussex in the UK.
"If you consider black holes within only general relativity, one can show that they have a singularity in their centres where the laws of physics as we know them must break down.
"It is hoped that when quantum field theory is incorporated into general relativity, we might be able to find a new description of black holes." //

the finding could have interesting implications for our attempts to square general relativity (on macro scales) with quantum mechanics (which operates on extremely small scales).
Black holes are thought to be key to this undertaking. The black hole singularity is mathematically described as a one-dimensional point of extremely high density, at which point general relativity breaks down – but the gravitational field around it can only be described relativistically.
Figuring out how the two regimes fit together could also help tp solve a really thorny black hole problem. According to general relativity, information that disappears beyond a black hole could be gone forever. Under quantum mechanics, it can't be. This is the black hole information paradox, and mathematically exploring the space-time around a black hole could help resolve it.