Scientific American astronomy editor George Musser explains.

This question really has two parts. First, how was matter able to get out of the big-bang singularity? After all, physicists describe a black hole singularity as a pit into which material flows but from which it cannot escape. Let us leave aside the fact that singularities are an idealization. The basic point is that the universe was born with a tendency to expand, which overcame the tendency of matter to collapse. According to relativity theory, space does not like to remain static; for all but the most special cases, it either expands or contracts. But why it initially chose the former is still a mystery.

In some ways, you can think of the universe as a black hole turned inside out. A black hole is a singularity into which material flows. The universe is a singularity out of which material has flowed. A black hole is surrounded by an event horizon, a surface inside which we cannot see. The universe is surrounded by a cosmological horizon, a surface outside of which we cannot see. (A crucial difference, though, is that the event horizon is fixed whereas the cosmological horizon varies from observer to observer.)

The second part of the question is: Why didn¿t matter in the early universe collapse into black holes? After all, physicists say that if you squeeze matter to a high enough density, it will collapse into a black hole, and the density of matter in the early universe was extremely high. The answer is that black-hole formation actually depends on the variation in density from one place to another--and there was very little variation back then. Matter was spread out almost perfectly smoothly.

In fact, cosmologists usually turn the question around. The fact that the universe did not recollapse into a swarm of black holes is evidence that sharp density variations did not exist (or were extremely rare). This lack of sharp variations, in turn, is evidence for the inflationary model that most cosmologists today accept.

Robert J. Nemiroff, assistant professor of physics at Michigan Technological University, responds.

First of all, it is not really known whether or not the universe started from a singularity. Our measurements can take us back only so far; ideas about the nature of the cosmos at the start of the big bang are mostly unproved conjecture.

Second of all, the concept of a black hole is only one type of solution to Einstein's General Theory of Relativity, our best current theory of gravity. This reading of general relativity--known as the Schwarzschild solution--is thought to give an accurate description of the gravity near an isolated, nonrotating black hole, as well as the 'normal' gravity near the earth and throughout our solar system.

But other solutions to general relativity are known to exist, including ones that apply to a whole universe. These alternative solutions typically assume that the early universe was perfectly uniform so that there were no places for black holes to form, even if the density were so great that particles were "cheek by jowl." The most popular class of general relativity solutions applying to the entire cosmos are known as Friedmann-Robertson-Walker solutions. These formulations appear to describe correctly our expanding universe; that is, they demonstrate how objects not held together by local forces (such as the electromagnetism that bonds atoms in molecules or the gravity that keeps the earth intact) stream away from one another in a predictable manner.

Still, there is room in the theories for some of the matter in the universe to be hidden in black holes that might have formed from local, unusually dense regions in the very early universe. These black holes could conceivably contribute to the large amount of dark matter that exists in the universe. Astronomers are therefore diligently searching for these objects. In one scenario discussed by Jeremiah Ostriker of Princeton University and his collaborators, black holes as massive as one million times the mass of our sun might be common throughout the universe and still be nearly invisible. Although other black holes might come out of some big bang models involving quantum mechanics, a common expectation by cosmologists is that only elementary particles survived these early epochs of our universe.