Q&A: Black Holes

Q:
If matter was so dense at the time of the Big Bang, how come it didn't form a black hole?

A:
Similar questions have bothered eminent scientists from Newton to Einstein and beyond. The basic reason that most of the matter in the Big Bang did not collapse immediately into a black hole, is that the universe was expanding from a very hot state. The expansion, plus the pressure of the intense radiation in the very early phases of the Big Bang, had the effect of slowing down the collapse of overly dense regions until much later. How much later? Astrophysicists presently estimate that the first stars and galaxies formed somewhere between a hundred million and a billion years after the Big Bang. It is still an open question as to whether large numbers of very massive stars heavier than a few hundred solar masses formed at this point and imploded to make black holes. If this happened, then massive black holes might be the mysterious dark matter, and most of the matter in the universe might be in the form of black holes.

References:
J. Silk, "A Short History of the universe" W.H. Freeman, 1994.
M. Begelman and M. Rees, "Gravity's Fatal Attraction", Scientific American Library, 1996.