Question

what is there at the back side of the black hole?

Answer 1

there is nothing outside

Answer 2

Blackholes (only eternal blackholes which do not emit Hawking Radiation) do have a 'backside', though not in the conventional way. These backsides are called white holes and are hypothetically formed by extrapolating the time space graph. But all of this is just theory. According to Hawking 'backside' of a normal blackhole formed due to gravitational collapse is again a blackhole in the space-time.