Black holes cannot be observed directly and therefore cannot be "discovered". The indirect evidence for two kinds of black holes is now overwhelming. Those of a few solar masses produced by supernovae and much larger ones at the center of some galaxies.
The existence of bodies with gravitational fields strong enough to allow nothing to escape has been a topic of speculation for hundreds of years. Einstein's general theory of relativity (published in 1916) predicts just the kinds of object we are now inferring.
Perhaps the first object to be generally recognized as a black hole is the X-ray binary star Cygnus X-1. It's effect on it's companion star suggested as early as 1971 that it must be a compact object with a mass too high for it to be a neutron star. (That was 2 years after the American astronomer John Wheeler coined the term 'black hole').
How do they
form? Perhaps if a star was large enough and it collapsed, maybe
nothing (not even light) could escape from it. All the matter of
a star (even it's energy) would be drawn into a denser and denser
single point. At first Einstein thought that this couldn't be
possible! He though something in nature would prevent this. The
"Cosmological Constant" would have to prevent this. He
later regretted this and said it was a huge mistake! He found in
his equations pointing toward difficult possibilities: the
expansion of the universe and the collapse of matter into an
infinitely dense point. Einy buddy, your a GENIUS! Have a little
more faith in your mathematics! In recent years we've already
proven the the universe is in fact expanding and in a few years
i'm sure we'll prove the existence of black holes. Some believe
that if two black holes connected in some mysterious way it will
become in essence a worm hole.
Formation:
If it emits no visible light how can we find them? Black holes devour everything around them, no matter what. Planets, stars, even whole galaxies! We can tell it's there because it is feeding on other energy masses that we see being dragged into the center. Some black holes that astronomer's have detected are upwards of 10 solar masses (1 Solar mass = Mass of the sun). We can also see stars that are revolving around invisible points of incredible mass that are obviously there, but we can't see.
What would it be like to enter a black hole? This is a place where all of Einstein's and Newton's theories and equations fail. A singularity, or a worm hole? What ever happens there are a few things we know...
As you
approach the center you'll begin to stretch. On Earth there is a
slight difference between the gravity pulling on your feet then
on your head. Your body is slightly being stretched out. Well, in
a black hole it is much greater. Nothing can survive this
stretching. You'll be stretched and crushed into a singularity. Spaghettifacation as cosmologists
like to say. That is what we know of, but what about worm holes?
Skipping around from universe to universe through these worm
holes is not to unreal. Could this be the fastest way to travel
through space and time?