Black HolesBlack Holes are theoretical objects. A black hole is an object which has such high gravitational pull that not even light can escape. Hence it is black. Which makes it pretty hard to observe directly.
Black Holes may form as the result of the collapse of a very massive star at its death. Just as neutron stars form during a supernova explosion of a very massive star so do black holes. But in the black hole case the initial star was so massive that nothing could stop its gravitational collapse. All the matter of the star's core is crushed to an infinitely small point, a singularity.
A black hole is shielded from the outside world by what is called an event horizon. This is a sphere around the black hole where light can no longer escape. Everything that crosses the event horizon is gone forever.
Despite the blackness of black holes there is evidence of their existence. If a black hole forms near a star it may get gobbled up by the black hole. And this can be seen by Astronomers. Some believe that many if not most galaxies harbor supermassive black holes in their cores. These black holes have gobbled so much matter that they have the mass of billions of suns.
What would it be like to enter a black hole? Not pleasant. First, as you approach the black hole the difference in the gravitaional pull on your head compared to your feet (known as tidal forces) would rip you apart. But suppose you survived that. Once you cross the event horizon there is no turning back. Then the only thing to do is avoid the black hole itself at all costs. If you run into that singularity it will crush your body right out of existence.
If people from Earth were watching your journey into a black hole they
would never see you cross the event horizon. Albert Einstein's theory
of General Relativity says that as you approach a black hole your time
slows down. The closer you get to the black hole the more you appear
to be in slow motion as seen by Earth. Eventually you appear to be
frozen in time as you cross the event horizon. You would notice
nothing different whatsoever. If you were to change your mind right
before crossing the event horizon and return to Earth you would find
it in the very distant future. You've become a time traveler! Neat