LIGO detects most massive black hole merger to date

> the 225-solar-mass black hole was created by the coalescence of black holes each approximately 100 and 140 times the mass of the Sun.

Does this mean that 15 solar masses were converted into energy? Because that's a LOT of energy.

Into what form of energy is that mass converted?

Maybe all of it is gravitational waves?

I don't think much else would escape the black hole environment.