It sounds very far-fetched, but there is possibility that colonies of aliens may be living on planets within certain black holes, a Russian scientist has claimed.
According to Professor Vyacheslav Dokuchaev from the Russian Academy of Sciences, some black holes have a complex internal structure that allows photons, particles and planets to orbit a central singularity -- the region in a black hole where space and time become infinite, the Daily Mail reported.
However, at the centre of certain black holes, and under the right conditions, there is an area where the fabric of space and time exists once more, Prof Dokuchaev said.
If a charged and rotating black hole is large enough, he claimed, it can weaken the tidal forces that are beyond the event horizon -- the point where nothing, not even light, can escape a black hole's gravity.
Scientists have long known that photons can survive in stable periodic orbits inside such charged black holes.
However, Professor Dokuchaev said that a black hole's inner Cauchy horizon -- the area where dimensions switch back again -- can also accommodate particles and even planets.
These manage to exist without ever getting sucked all the way into the black hole and would derive light and heat from the orbiting protons and from the energy of the central singularity, he said.
And these conditions could prove self-sufficient for alien life, he speculated.
He said: "This internal black hole domain, hidden by two horizons from the whole external universe, is indeed a suitable place.
"Advanced civilisations may live safely inside the supermassive black holes in the galactic nuclei without being visible from the outside."
Earlier this year scientists discovered that the black hole M87 is almost twice as big as it had previously been thought.
The black hole at the centre of the Milky Way is 1,000 times smaller than M87, which was observed in the largest and most distant galaxy some 50 million light years away.
Scientists believe that it may have formed as a result of hundreds of smaller black holes merging into one at some point in the past.