The rapidly increasing knowledge about binary black holes is due to LIGO/Virgo gravitational wave observations started in 2015 with the discovery of the first coalescing binary black hole GW 150914. Presently, several dozen more such binaries have been detected, with the registration rate of about ten sources per month in the ongoing O3 LIGO/Virgo observations. A review of the observed properties of the LIGO/Virgo binary black holes will be given. Their possible formation mechanisms will be discussed, including the standard astrophysical scenario from the evolution of massive binary stars, via dynamical captures in dense stellar clusters, and the formation from primordial stellar-mass black holes.