There are likely millions of black holes in our galaxy, and we will probably never know where they are

If you fell into a black hole, you would never escape . Scientists call this point of no return around a black hole the event horizon. 

If you fell into a stellar-size black hole, you could turn into human spaghetti. If you fell in, your feet would immediately get pulled way more than your head, and you would stretch — a lot. This super stretching effect is called spaghettification. 

Einstein didn't discover the existence of black holes – though his theory of relativity does predict their formation. Instead, Karl Schwarzschild was the first to use Einstein's revolutionary equations and show that black holes could indeed form. 

Black holes literally pull the space around them. 

There is a supermassive black hole at the centre of our galaxy. The black hole at the centre of the Milky Way, Sagittarius A, is more than four million times more massive then our sun.  

Black holes evaporate over time. Hawking radiation disperses a black hole's mass into space and over time, and will actually do this until there is nothing left, essentially killing the black hole. This is why Hawking radiation is also known as black hole evaporation.