In the world of theoretical physics, black holes hold a special position. A growing amount of evidence exists that all but proves that there are black holes out there. This isn't easy for an object that can't be seen, because the gravitation is so great that not even light can escape. Even more interestingly, there is a good amount of data showing that micro-black holes may, and probably do, also exist.
A black hole is hypothesized in Albert Einstein's relativity equations. At its most basic level, a black hole is an area of extremely intense gravitation, capable of warping the fabric of space time. It is believed that normally black holes are formed when a super massive star, several times more massive than our sun, collapses. These black holes have a mass of the original star and any matter that was sucked into the black holes. Though the singularity, or center, is tiny, the object is enormously massive.
In comparison, a micro-black hole would be much smaller. It could have the mass of the earth. At first glance, this would not be possible as the originating body wouldn't have a large enough mass to collapse into a black hole. To see how they could form though, a person only needs to look at the beginning of it all, as far as it is currently hypothesized.
The big bang
While theoretical, the big bang is accepted as the likely beginning of the universe. By this theory, at the very beginning, there was no gravity. Atoms didn't even exist. All matter was plasma. Gravity and all the other forces separated early on, after the universe exploded outward. The flux means that when gravity first separated out of the early universe, the plasma could have been incredibly dense in some places. There would have been no weak or strong nuclear force yet, so particles could have been packed every close together, unlike later on when the forces existed.
This makes a huge difference. If the atoms in the sun didn't have the restrictions of the forces that exist today, it would have the same mass that it does now, but would probably be far denser and smaller than the earth in area. When gravity came into existence, the rules that govern black holes would have come into play.
At the moment gravity separated out of the early universe, massive objects could collapse into black holes. They wouldn't need to have a great size, only a great mass. The singularities would be tiny, perhaps as small as the nucleus of an atom or less. However, the total mass would be gigantic. An object the size of the earth could have become a micro black hole, because there wouldn't have been space between atomic particles. This is still a great mass, though not on the scale of stellar mass. It is hard to imagine, but the earth could have become compressed to an object smaller than the period at the end of this sentence.
In the very early universe, the conditions were prime for creating very tiny black holes. There is some hypothetical evidence that miniature black holes do exist. This could explain some of the strange cosmic gravitational abnormalities. Like all black holes though, they will be very hard to actually see and measure, if they do exist.